Traditionally the Supra was always a 2+2 (my dad had an '84 when I was a little kid) while the Z was a 2-seater. I guess that the Z did gain a back seat once the 280ZX 2+2 came out, and in their final iterations both were big, heavy twin turbo things with similar sticker prices though; so maybe the new Supra will become a 2-seater like the Z has become.

When it comes out I'm sure it'll be similar in execution to the 350Z (i.e. based off a corporate small rwd luxo sedan platform) regardless of whether or not it sprouts a vestigal back seat.