Vaporeon.
Pros:
High HP
Can be used as a support, cleric, and special attacker
Can tank special attacks
Higher base stat total
Is better offensively
Cons:
It will almost never hit first
HP isn’t the same as defense, and it’s defense is mediocre at best
It can’t hit special tanks, making its role much more of a cleric/support, which is not ideal for in-game
Poliwrath:
Pros:
Stats are balanced
Can hit with both special and physical moves
Faster than Vaporeon
2 STABs
Cons:
None of its stats shine
Water/Fighting is weak to 4 types in Leafgreen, more than Vaporeon
It will not hit as hard as Vaporeon would specially using physical or special moves
There is a very small move pool of moves it can learn by leveling up (the same could be said for Vaporeon though)
In conclusion, I would use Vaporeon for its bulkiness and great special attack stat. If you are looking for something more balanced, maybe go with Poliwrath. I still think Vaporeon is better though.