Either one will be nice. red bellies are the most common ones since can be breed in captivity easily now. it's also the most skittish one compare to others in the pygo family. however, if provide proper care and condition they can be very nice too. they are the smallest so you can probably have 12-15 in 180g (not the 20g per fish rule but as long as your filtration can keep up, over stock it

) you can also get wild reds (super red I think)
Piraya is the most colorful and from what I have read that their color doesn't go away with age like other pygos do. they are also the the largest and most expensive and cannibalistic compare to other pygo. for 180g I'd say 3-5 for life.
Caribe is one of the less skittish compare to red bellies and more aggressive. cheaper and smaller than Piraya but still more expensive than red belly. don't think is their season now so hard to find them in couple inch in size. 8-12 for 180g
Ternetzi is similar to red belly I believe but yellow instead of red, hard to come by and should be less skittish and more aggressive than captive bred reds. 8-12 for 180g
There are few websites have very good information on them
These three are translated from French
Red Bellies
Caribe
Piraya
http://www.piranha-info.com/default.php?lang=en&id=page_1
and of course
http://opefe.com/
If you couldn't find the them at lfs, then you can try to order them online.
Aquascape is great, so is
Shark Aquarium
If I miss anything or put down incorrect info, please feel free to add in and correct me.