Devantay,
Don't bring your bro' down for a bachelor party. He'll want to cancel the wedding.
The choice is simple: For better/prettier beaches, better hotels, a national park right next door etc. the choice is Quepos/Manuel Antonio. M.A. is actually about four miles down the coast from Quepos; north of Quepos are also several fine little beach towns.
Parque Nacional Manuel Antonio was Costa Rica's first national park and is beautiful, despite high tourist traffic. Playa #3 is a Hollywood movie-perfect tropical beach; you have to walk there, maybe even fording a small river or two. Trails are fairly well marked, and certified guides can be hired for a few bucks an hour. Yes, you'll see cinnamon ringtail monkeys, maybe squirrel monkeys, caymans and other critters. Many bars/restaurants are clustered around the park entrance; I like Dos Gemelos for cold beer and good sandwiches.
Hookers are available around Quepos, but be sure to check I.D.'s, as reports of under*ge hooker wannabees have been lately circulating.
I've never stayed in Jaco, but the afternoon/evening I spent there was less than stellar. Yeah, it gets hot and humid particularly during the dry season (November-April), but shit...the coastal regions of Latin America are all hot and humid.
DGD's advice above is exactly correct. I would add that hookers occasionally work the casino at the Best Western Kamuck in Quepos, although as DGD says, the action is sporadic at best.
One place he forgot to mention, and which I just thought of is Bar Rosa, north end of Quepos. If it's still there, you can bang a hot 3 - 5 (hahaha) for around $50.00.
