Okie,
We don't want to hijack this thread but I'll offer you my own quick take on your situation which I'll bet most vets around here will concur with.
If you want the most P4P or non-P4P nightlife options, then Jaco will definitely be a more fun-filled location. Jmac is absolutely right about having to bring your own female companionship if you stay in MA and you won't have much time to pull that together before you leave SJ. Though it sounds like not having a chica with you might not be the end of the world for you, you'll at least have the option of finding a reasonably priced local chica if you stay in Jaco. The beaches in MA are much nicer than the one in Jaco, but you could always hop on a cheap local bus (just a little over an hour each way) for a day trip to MA from Jaco. If local buses seem too adventurous for you, you could also just hop in a cheap local taxi and drive out to several decent beaches just a few kms outside of Jaco.
For salsa dancing, probably the best and simplest thing for a newbie to do is just go to the Key Largo. Its right across the street from the Blue Marlin and you'll probably be spending a lot of time in there anyway. They have live music bands (usually salsa) starting around 10PM each night. There are lots of ready and willing chica dance partners. And, because most of the other gringos in the place are as lousy dancers (or worse) than you may be, you don't have to feel at all self-conscious about how well you do (or don't do). Sure the girls may want to go back to your hotel with you afterwards, but most will not have any problem at all with a guy who just wants to dance (particularly if you buy them a drink or two, spread it around and don't take up all their time so they can find another paying customer).
It sounds like you're thinking of exploring non-pro possibilities. I'd advise you not to get your hopes up too much with so little time in town to work with, ESPECIALLY if you don't speak much spanish. Brush up on whatever you can before you go.
If you don't want to make mongering the focal point of your vacation, then I'm probably one of the odd-ducks around here that you'll want to probe for ideas. Like you (and many others here), I like to get outside the Gulch and SJ. However, this being your first trip you don't really have to worry about any shortage of non-mongering things to do as day trips using SJ as your base (leaving all your nights free for mongering or salsa dancing in the capital). For a "sensory overload" of ideas, check out my posts in these threads:
http://www.costaricaticas.com/phpBB2/viewtopic.php?p=177762
http://www.costaricaticas.com/phpBB2/viewtopic.php?p=45003