So far, the novel has been hinting that despite Ise's growth, he is still weaker than Vali since Vali has been growing as well, at the same pace if not faster, which makes sense since if two beings with different base stats, one being superior to the other, had the same amount of growth, the superior one would remain superior and the inferior one, inferior. But, if you consider other factors, then things may turn out differently.
Ise's abilities/skills/equipment up to latest in chronological order.
Normal mortal
Half Devil w/ human base
Boosted Gear Initial form
Boost (10 seconds interval)
Promotion
Boosted Gear upgrade (vs Reynael/Reynare)
No proficency in magic or close combat
Dragon shot
Dress Break
Gift/Transfer
Training to master the gear through random tests without any proper planning
Imperfect Balance Breaker
Right dragon arm
Simultaneous Boosting
Limited Dividing
Ascalon
Incomplete Juggernaut Drive
Longinus Smasher
Simulatneous Boosting/Dividing
Complete Balance Breaker
Reading mind of boobs (Forgot the name...can't believe I typed that)
Chichigami's protection (That was a one off joke but included for completeness)
Fire breathing (Underground train tunnel in Kyoto/Heroes Faction against shadow sacred gear user)
Some proficency in close combat and magic though very little for the latter
Rule Breaker Forms
Dragon base body
Unpredictable(growth/personality), cautious, low capacity thinker
Very good imaging ability even outside of situations that is not related to his hormones.
Well, that's quite a hefty list I'm remembering off the top of my head and it has been a few months since I last touched the novel so you'll have to pardon me if I left out anything.
Now for Vali.
Guided and trained by Azazel, who has lots of knowledge and information on Sacred Gears
Complete Balance Breaker
Supposed 'complete' Divine Dividing form
Successor of original Lucifer blood
Genius
High proficency in magic
High proficency in close combat
Melee ranged divide
Half Dimension (It was the AoE divide if I'm not wrong)
Targeted divide
Mastered Juggernaut Drive
Growing at a very fast pace
Arrogant, reckless, tactician
That's all I can remember since Vali has more or less been stated as 'A very strong being that is growing at a frighteningly fast pace' despite the growths of the main cast.
Now that we've more or less threw everything out, we can more or less 'decide' who may win should they face off.
From the top, Ise does not have the experience in fighting that Vali had mainly due to their lifespan difference. True, most of his fights had always been against people stronger than him, but it was all more or less against his will. Everytime that he is thrown into a fight, he participates half heartedly until something spurs him on. This mindset will subconsciously hinder his growth to some extent. Also, unlike Vali, who had been training under Azazel's guidance, Ise has been getting strong through training directed by Rias, who herself is also inexperienced and not that knowledgable in Sacred Gears, compared to Azazel. Also, now that he has a dragon base body, he is now subsceptible to things dragon and devils are weak against and unlike Vali who can use his immense magical energy and sacred gear to negate and reduce some of the damage caused by their banes, Ise will not survive. Ref. Vol 11. Vali was able to hold on against Samael's poison but Ise just got OHKOed and that was only when he was part dragon, now that he is half dragon, the poison would be roughly twice as effective.
Ise's main 'selling point' is that his growth is unpredictable, he does things that would stun you shitless in the middle of a fight. He also suffers from the sickness that all main characters have in that the peak of their emotions(usually anger) can somehow be coverted to strength of will, or just pure strength, and use it to defeat someone who is way stronger than them. Furthermore, he also has the magical power of friendship, which will somehow draw out powers hidden deep within, something Vali, in his pursuit of strength, will never be able to do, ironic, isn't it. Also, his rule breaking forms all have their strengths and weaknesses which allows him to become a much more balanced fighter that can adjust to situations. However, the transformation between forms have a minor delay which will leave an opening, one which Vali will defintely take.
Vali on the other hand is a battle maniac who has had plenty of experience. He gladly throws himself into battle against anyone, anywhere. No opponent is too weak for him to fight and no opponent is too strong for him to defeat (With some exceptions anyway). A pure seeker of strength who will take every opporunity he has to become stronger than he was a second ago, and if there isn't any, he'll make it so that there is. Being a devil that inherited the original Lucifer blood blessed him with high magical power, one which he has high mastery in and also being a natural genius. He is also guided by Azazel, who knows how to bring out the strengths in someone. Aside from being strong in a fight, he is also able to fully utilize his abilities, such as dividing projectiles into nothing before they hit him, or to something minor in the event that it will hit him, and then proceed to blow off the already weakened projectile with his immense magical power.
Sadly, not everything is sunshine for Vali as has strength has made him arrogant. Not only that but he is also reckless and a pretty passionate person deep down as well. Also, he is quite reliant on brute force, blasting away anything that he cannot outmaneuver or just taking damage which he takes he can negate with his magical power. Despite being a decent tactician, his arrogance, passion and recklessness will ultimately cause him to err in his judgement, giving Ise an opening which he will defintely not miss out on. Unlike Ise, who found a way around Juggernaut Drive, Vali chose to remain and use the Juggernaut Drive. Despite all his strengths, he is unable to maintain it for long and can only use it without any repurcussions assuming that he has magical energy to 'feed' the Drive, whreas Ise can use his Rule Breaker forms for a long period in time.
To summarise. In the event of 1 v 1, it comes down to terrain and who gets to their 'strongest state' first. If Vali is able to get high enough into the air while being out of range for Ise to get a decent aim at him, he can easily use Half Dimension and then blast him away with magic. Even if Ise goes rook to absorb the blasts, he will wear out sooner or later, or even worse, his Balance Breaker will give way, all of these before Vali's mana pool runs dry. Even more troublesome is the fact that his Balance Breaker time is consumed greatly every time he changes forms and uses boost whreas Vali does not suffer as much from using his abilities in Balance Breaker. If Vali drags this into an endurance match, which is Ise's specialty, he, Ise, will ironically be at a disadvantage.
However, because of Vali's arrogance and Ise's unpredictable factor, things may turn in favour of Ise right before the decisive moment. If he times it right and uses the Divide + Boost combination in sucession, he can overpower and defeat Vali then and there since whatever Vali was going to use to finish him, will fail and if he is unlucky, blow up/go berserk on him.
I know that I have not touched on things that I have listed out but some of them are really self explanatory, Ascalon for example. Ise has barely used it and everytime he did, it wound up in someone else's hand at the end of the battle, mainly Kiba or Xenovia. That and it's about 12.30 am by the time I finished this post so yeah.