i've had a few good friends with mkivs. they're awesome cars. here's my take:
-the supra is way easier to mod because of the aftermarket support. i don't think the impreza will ever have the kind of aftermarket support that the supra has now.
-as for potential, i'd say subaru doesn't focus on the same things that toyota did with the mkiv. as far as drag racing, there's no way the STi would ever touch the supra (i assume we're talking about cars with plenty6 of mods here). in an autocross or rally, the tables turn.
-pricing is something i'm a little bummed about. since the rice and the ridiculous came out, supras have gone through the roof. it used to be you could find a mkiv tt in good shape for well under $20K, now it seems like they're all ~$25K. they're the car of the moment, so if you want one i'd advise waiting a year or two. resale will screw you if you buy one right now.
-another thought on pricing... i see a lot of supra owners that have a flawed theory on value. their formula is:
(original purchase price) + (all $ spent on mods) + (labor, even if the did it themselves).
i've seen plenty of people asking more for their supra than what they put into it. i don't get it.
-as for aesthetics, i think the supra is gorgeous. i have to be honest though, i've never understood why people ask about looks in posts like this. if you like it, you like it; who cares what other people think.
-there are other factors. snow. insurance. mileage. and all mkivs are used at this point. these are things to think about if you're serious.
i dig the supra. it's way faster than a z32, way more reliable than an fd, way more affordable than an nsx, and way more available than an r34. it's definitely the cream of the japanese supercar crop.