The Wrx is far from being a weak car straight from the factory. I would have killed to have that for a first car. Or second or third or fourth...

I would recommend against getting a used Sti because:
A) The same reason the previous poster mentioned
B) You are probably not ready for it. Not being an a** just being realistic.

I'm a decent driver with more experience than you've been alive and it took me probably 3 weeks or so before I was really confident enough to push the Sti to the (car's) limits. Do not read that as me being some sacless wuss because that's far from the truth. I've had many people swear in my car when I've taken them for a ride.

Start out small(er). The Wrx is a great first car and one that is probably more forgiving on many fronts than the Sti.

To address your questions on the differences between the Wrx and the Sti:

It's like the diff' between a Mustang Gt and a Cobra. Yes they are the same car but night and day difference. Wrx and Sti even more so.

The Sti has upgraded brakes, suspension, wheels & tires, transmission!, engine (2.5l 300hp vs 2.0l 227-235hp), turbo, interior, etc, etc.. I couldn't believe the difference handling and performance wise between the two after I drove the Sti.

If you've only had your license for a few months awd is going to blow your mind.

G/l in your search.