I would choose Starmie over Poliwrath personally. I'm more of a Special Atk/Def person. Also, Starmie has a greater ability to learn moves, giving you the chance to create a better moveset than possible for Poliwrath.
I have also experimented with battling the two. Starmie can beat Poliwrath without using type alignments (Starmie's Psychic vs. Poliwrath's Fighting)
On the other side, Poliwrath can learn a lot of good physical attacks. I have one with Dynamicpunch, despite its low aim. XD
To decide, ask yourself: Do you battle better with physical or special moves?