i'm american (usa), and i've never been to south/central america, but i have traveled a bit. i think it all depends on what you're looking for. i've been in fairly wild/on-your-own type places (mongolia, nwfp pakistan, for example), and i've traveled in "super civilised" places too (western europe, etc). they each hold enjoyment, but very different kinds of course. what you need to do is ask yourself what kind of experience you are after, and then look at the potential programs each place offers. you said you are doing it through a company, so your options may be limited from the get-go. i hate those companies, usually, i think it's much better to travel completely on your own or with 1-2 good friends, but different people are different, right?

each place should offer plenty of ancient stuff (aztecs, olmecs, etc), plently of nature (rainforests! deserts! etc), or plently of modern amenities (caracs, san jose, puerto vallarta, etc). just think aboput which of thoise things are most interesting to you, then do it! also, think about which place you are a least likely to go to again.. i mean say you live right across the border in texas.. you can go back to mexico whenever.. or maybe you have a family friend in brazil.. you can hit up venezuela easily in the future.

good luck, you'll have fun where ever you choose!