There isn't a real horsepower difference per say but the tubes change the horsepower/torque curves. The long tubes move the horsepower/torque curve up on the high end and the shorties move it up at lower rpms. I might be mistaken about all of that but if so someone will fix me. Both will give you more than stock but in different ways.
The under the curve power and torque are typically improved nicely. The peak #'s are usually similar. I did the shorty to longtube swap on my old 302 combo and I felt a low end gain. Sound alone was worth the swap
I run bbk longtubes and they're really nice pieces BUT the collector is restrictive and needs to be opened up.
Some things to think about when going between shorties/longtubes.
Longtubes will give you a .05 second gain in the quarter mile (per one example) over shorties. Longtubes cause less ground clearance, the inability to use the stock h-pipe so you have to buy one to match which equals more $'s, more interior heat, harder to install, some steering shaft clearance issues, harder clutch installs.
Shorties won't be quite as much power (.05 seconds less per example) but have more pros than cons.