I own an Evo IX MR, and I'd still suggest getting the '09 WRX simply because the Evo isn't the type of car you want to buy preowned unless you really know the history behind it.
I also don't really see the upkeep as being all that expensive (I bought the car new in April of '06 and the only thing I've HAD to do as far as maintenance is routine oil changes - but then again I don't put a whole lot of miles on it), not for the performance it gives you. Basic mods are inexpensive, and can give you phenomenal returns if done correctly; just like with any cars of this type however, when you get people throwing mods on left and right without any research done, and with questionable install work, you start running into problems. Personally, I've paid a little more to get quality parts and professional installation/tuning (Buschur's done all the work that I haven't done myself) and the car has had ZERO issues.
Still though, this isn't necessarily true of most Evo owners, which is why I said unless you know the car's history I'd stay far away from a used example. Pay for the new WRX, and while you may not have the raw performance of the Evo you'll be assured that you won't be throwing money at it constantly to fix other people's problems.