you do lose a little space, stagea is 940mm between the strut towers, legnum is 890mm, and the interior space is a bit smaller, you notice it in the 2 front footwells the most.
aftermarket support is there, RHDjapan sell a shiteload of gears, and alot of manufacturers like tein, fujitsubo, pro shop iida, make parts for them.
cheaper parts is one downside.. but then it depends what you're after and want to do, a VR4 dump/front pipe setup is cheaper than that for a RB25/26, and a cat back exhaust is much the same.
factory 5 goodness.. do you mean a 5spd gearbox? because you can get a 5spd tiptronic, or a 5spd manual (VR4 manual's are alot more common that stagea manuals too...)
the better interior is in the eye of the beer-holder too.. personally i love my leather, carbon fibre, and recaro's over anything the stagea offers.. however there is also tan/black leather, straight black leather, suede, black/red cloth, and S1 stagea style cloth too.
in the end, its up to what you like.. a smaller nimble little car with plenty of torque that will run low 14's out of the box, or a larger car with more room, more available parts and support and a different look.
i would have bought a stagea myself if kylie didnt already have one.. now we just have a sexy black S1 rs-4 stag, and a sexy super vr-4 legnum in the driveway.
i guess the real question is... would you prefer people to say "nice volvo!" OR "nice magna!"