Slowbro is better.
Compare:
Stats:
Slowbro has higher hp.
Poliwrath has higher attack.
Slowbro has higher defence.
Slowbro has higher special.
Poliwrath has higher speed.
In total slowbro has more stats greater than poliwrath.
Typing:
Both are water type, but slowbro is also a psychic type while poliwrath is fighting type.
Slowbro is better here as well because Psychic was the best type in gen 1, having an immunity and multiple resistances but only one weakness, the almost non existent bug.
Poliwrath however being a fighting type is weak to flying and psychic, and gets resistances to bug and rock. Fighting is also resisted by 4 types, and ghost types are immune to it, while slowbro's superior psychic is only resisted by psychic with nothing immune to it.
Slowbro can also use both of its stab moves off its higher attacking stat due to water and psychic being special, while if poliwrath uses a water move it will be weaker because it has a low special stat.
Moves:
Slowbro gets a slightly better movepool, learning good special moves like blizzard, psychic, surf and fire blast.
The best physical moves poliwrath gets are submission, earthquake and hyper beam.
You will definitely want slowbro as it fares well against many gyms and the elite 4 as well.
Hope this helps.
