Starmie is the better choice overall. Starmie also doesn't need to evolve anymore.
Why is Starmie the better choice? Because Starmie has a great offensive typing, a great movepool, and better stats. Starmie can also play different roles: supportive, and offensive.
Poliwrath is inferior because it has worse stats, it's not really good at anything, and it doesn't have the best movepool either. The best moves Poliwrath gets for STAB are Submission or Brick Break and Waterfall. It only gets about 5 good moves, unlike Starmie.