MRTurbo SR20 'truck motor' is overrated. 3S has a better valve train and perfectly square engine (bore=stroke) design. Lots of aftermarket support doesn't make for a better motor.
SR20 is a square engine, 86.0mm x 86.0mm, same as the 3S.
The main reasons I would go for an SR20 over the 3SG is the Aluminium block on the SR makes it a lighter package, and you can get VVTL from the SR20VE Neo motors (BEAMS is only VVT, no lift control). For application in an MR2, I'd stick with the 3SG simply because I don't think Nissan have any decent FWD gearboxes to that will bolt to an SR motor. It's pretty hard to walk past the E153 when your aiming for decent power and it's a factory config....