The longtubes have been proven to make a few more HP, but I cannot tell you the physics as to why.
IMO: The few extra horsepower you get from longtubes is not worth the extra cost of those headers; unless you are trying to build an all-out car and want to squeeze every HP out of it possible.
Also, the longtubes are harder to install: Trust me when I tell you, putting headers on these cars is a huge PITA! (pain in the ass, in case you aren't familiar with the acronym). If you are going to do it yourself, I highly recommend you get a ratchet wrench for the header bolts; regardless of whether they are longs or shorties.