For me the P2, even in the long, is kind of a cramped bike. I've heard others say this as well. Since you're 5'8 (I'm 6'1 and gangly as an ape) you could probably ride the small though, but if you have a freakish reach like myself you may prefer the long.
specialized p.2, schwinn predator, to be built ss road/track machine.
definetly go with the long. if i remember right the only difference between the short and long is the top tube 15in and 18in or something like that and the wheelbase is a little different but im about your hieght and closer to 200 lbs and the long fits me perfectly and i wanted to get a short when i was looking to buy mine.... i have an 04 by the way.