drmojo--
depends what interests you more.
JACO: I've only been to jaco to surf the areas around it. didn't seem to be much more going on there than that. if you want beach activity, jaco might be better. if you want the chance to hook up with a group of horny girls-gone-wild type gringas getting drunk at the hotel bar, jaco will be better.
QUEPOS: I went to quepos to fish and check out parque nacional manuel antonio. not much going on there other than that. manuel antonio park is something pretty awesome to see. fishing was pretty good too.
GIRLS: as far as girls, I've never sampled working girls in either of those areas although a couple friends I was with went to the strip club/brothel in quepos and said it was not so great. there's also a gringo bar with working girls nextdoor to the best western, but the girls were 4-5s at best.
after hanging in san jose, I didn't have much motivation to look for hired company in quepos. all I could think of was getting back to the city
RAIN: finally, keep in mind that sept is still the rainy season and both jaco and quepos rain like crazy in the afternoon through the morning.