I may be competent person to speak about it so I'll try...
I am moderator for forum.subaru.pl and had been fan of the Subaru cars ever since I saw Impreza Turbo years back. I've driven most kinds of Turbo and STI models avalible here and have some driving skill (kind of rally-driver-in-training). I also own Escort Cossie small turbo with some modifications, rebuild from piece of junk in last couple of years.
The difference if HUGE. Cossie is much more demanding car for the driver. It's simply hard to drive. Most Impreza models are easy or very easy. Cossie is more agressive, has more of rally-car feel to me. Engine wresponse is very agresssive and nothing like smooth EJ20 boxer. But Escos is technically very dated now - the worse conditions the better will Subaru be in comparison. Although my Stage 1 290bhp Escos can be made to go around stage or track as fast as high powerd STI it's only in dry or little wet. On soked road or in snow/gravel it stands no chance at all. Cossie is also very high maitenance car, it does not nececarely brake down a lot, but you have to have things checked and serviced avery now and then, and with Scoobies it wasn't unusuall for people I know to go for 2k km trip just after track-day.
If I could advice something - It's better to have Cossie as demanding car for track, stage use and simply fun on winding road. But as a daily driver, commuter or simply A to B car Scoobies will be 90% of situation better. And Cossie adjusted to be as fast as modern car - with rose-jointed suspentions, hard damping taes, strong LSD's and so on will be even more nasty in everyday use, some people like it though
I drive mine whenever it's ready. :-D
One more thing is that you can buy parts easily for any Scoobie and with COssies it's becoming harder and harder - soon this may make more and more cars go off-road from lack of parts. There isn't any real arts support from Ford anymore. Just couple of days ago I got to know that brake pump assembly, brake pump repair kit and some other braking system elements are now shown as deleted on Ford systems.