Many of the guys on here already hit the nail on the head. The Legacy GT has more useable space (for say, 4 adults), is a little less "boy racer", and IMHO is of a higher quality that the Rex.
The interior pieces, console, gauges, door panels, etc. are all much more substantial and of nicer materials.
I really do like WRX's, but the Rex just felt kinda cheap.
I drove both, along with an STI, and purchased a LGT because it fit all my needs and it still goes like stink....