Well, with 100% compression the Prommy 120 springs are giving real sword type 56's 399-402 FPS with prommy tight bores.. But IIRC the piston and cylinder are shorter than average..
Get a 110 and a 120 just to be sure, you'll ALWAYS be able to find someone that can use an upgrade spring... so don't worry about having extra.
As a rule of thumb, think of the springs rating as meters per second.. convert that to FPS and it'll give you a
VERY ROUGH idea of what the spring will give you
110M/S = 360.## FPS (gives me about 370-390 depending on setup)
120M/S = 393.## FPS (gives me 380-415 depending on the setup)
135M/S = 442.## FPS (Haven't installed one of these things yet so I have no idea.. but I know it'll be over)
It boils down to this; If you want to get EVERY last bit out of the gun, go for the 120... but there's a chance you'll be over.. so get a 110 with it just in case
Also, if you open a gear box.. Have a chrono. It makes your live so much easier, the Xcortech 3200's are really good.. and pretty cheap too