Either are worth a trip,

Singapore is a really nice place, the zoo is one of the best I've been to, and there are plenty of touristy things out on sentosa, zip wires, dolphin encounters etc.
There are loads of really cool swanky bars and restaurants, haunted by the ex-pat community predominately, and they can be expensive, but it doens't have to be. You can get really good food for dirt cheap at the street vendors, chili crab is a local speciality I think. Singapore also runs a food hygiene rating like here, I think it's A-D rather than numbers though.

Bangkok is cheaper, dirtier, more chaotic, but some people prefer it that way - there's probably more to see, landmark wise, as there are various temples, the royal palace etc. Food is cheap.
It can certainly be sleazy there though, unpleasant disturbing kind of sleazy at times. But if you stay away from the worst of it it can be an amazing experience.

on balance it would depend who i was going with, the mrs- singapore, the lads- bangkok