This question doesn't really give enough information, so take this reply with a grain of salt.
Since Ice Punch is learnable in games prior to Gen VII (factoring out transferring Pokemon over), I'll imply that you're using Pokemon Showdown. This reply also applies to an all-offense moveset.
If you have a Fighting move and Water move in the moveset, as well as Earthquake, you'll already have decent coverage. Now, let's look at the moves in question...
Poison Jab - 80 Damage / Grass and Fairy coverage
Ice Punch - 75 Damage / Grass, Dragon, and Flying coverage (Ground is already covered by Water)
Ice Punch covers 3 types, compared to Poison Jab covering only 2. In addition, if they send out a Grass or Fairy type, you may want to be switching Poliwrath out anyways. Poliwrath can easily take a hit from a Dragon type and counter with Ice Punch, though. Now for some more numbers...
Poison Jab + Water + Fighting + Earthquake: Super-effective to 530 Pokemon (66% of all Pokemon)
Ice Punch + Water + Fighting + Earthquake: Super-effective to 625 Pokemon (78% of all Pokemon)
That's a big difference. Even if you run Rock Slide instead of Earthquake, Poison Jab lets Poliwrath cover only 574 Pokemon, while Ice Punch would cover 598.
In conclusion... Ice Punch would be the better option all-around. Movesets vary though, so this may not apply to you.