THEY ARE NOT EVEN COMPARABLE!!!!
Well I mean they are, but Starmie is a thousand times better. :(
Staryu being one of my all time favorite Pokemon automatically bumps up Starmie in this versus thread. I've never been a big fan of the Slowpoke line, in fact I tend to forget about them all the time. Starmie on the other hand is a Pokemon classic... at least to me! Plus Starmie has such a lovely speed and special attack along with a diverse move-pool that it's so hard to ignore. Although Slowking has a nice diverse move-pool and special attack too so I shouldn't forget it either, but it's just not a memorable Pokemon.
Needless to say Starmie wins for me. :)