They do use the same cylinder; I've got both. (And can cause problems; see below)
In all honesty; there's not really any point in going for a long inner barrel. I've put PDI 6.05s in both (A 205 and a 395mm for the Shorty and carbine respectively) and there's no noticeable difference between the two; so unless you just want the aesthetics I wouldn't bother going for an extended inner barrel.
Re; the cylinders, they come with a full length (i.e. non-ported) cylinder which caused me problems with the FPS variance:
As stock, The shorty would chrono at 320ish with no hop on a .20, but once I set it to a level suitable for .28s it jumped massively to 370+, whilst the carbine version had the opposite problem; with an M100 installed it would chrono around 330 with no hop, but once set would drop to around 270/280. (Both with .2s)
Putting in an M4 sized cylinder in both helped remedy this, with a much smaller variance between no hop and having it set for my desired BB weight; and both dropping with increased hop as normal.
Not sure if they're all affected in that way, but that's something to be aware of.
As for upgrades; it uses its own hop unit, so changing the chamber is a no-go currently. It also has its own built-in hop nub, so would need modifying to change that; only the bucking can be changed as standard. I've had good results with both G&G Greens and PDI W-Hold buckings.