I am semi in the market for these cars, so let me spill some of my research over here.
First of all, Evo vs. STi is all preference. While they are very different cars they perform in a similar fashion -- in pretty much all aspects. This includes straight performance/handling but also reliability as well. There was a significant issue with the 08-09 STI's that was causing a lot of engines to go on stock cars. This issue was confined to cars produced before September of 08 though, so a 2010 will be fine. X's had a tune problem very early on but this was recalled, reflashed and fixed.
Outside of those small issues I haven't heard of many complaints for either car.
To be very honest with you I flip-flopped between both for a long time, pretty much changing my opinion with the winds. It really comes down to things like what interior/exterior she likes better (I prefer the Evo exterior but STi interior), which sound she likes better (the generic evo sound vs. the boxer growl) and all of those tiny little things that make a car unique.
Also, both of these cars respond insanely well to light modding and can be very fun and very unique without a whole lot of money or time invested.
I'm assuming you know the forums, but in case anyone else has the same questions/wants to do some research:
Subaru:
IWSTI.com: Subaru WRX STI Forums
North American Subaru Impreza Owners Club
Evo:
EvoXForums.com
Mitsubishi Lancer Evolution | Reviews, News, Parts, Owners - EvolutionM.net
edit:
also, this kid has done a **** ton of modding to his car but I love it so much (and how it sounds) that it still keeps me thinking about the current gen STi
http://forums.nasioc.com/forums/show....php?t=1791743