Something I'd like to point out. The Empire has had control for twenty years or so. In that time there haven't been any major conflicts. Training is good, but most TIE pilots and the like just haven't had much real combat. Most have served their tours, left the military and moved on, while new ones come in, train, serve, and leave. Sure, there's been counter-smuggling, interdiction, and other operations, but nothing like fighting vulture droids and tri-based fighters at breakneck speeds in the middle of a hundred ships all trying to kill each other. So, they've probably gotten rusty, ost the edge. 

Rebel pilots tend to be motivated, volunteer, fighting for a belief, not a paycheck. Usually the best have some background in flying, racing, or something, so they're likely to be decent on the stick. This was noted by Grand Admiral Thrawn at least once, especially in comparison to Imperial pilots, especially when pointing out the effects Vader and Palpatine had upon troops in combat (through Battle Meditation). But the Imperials still have the weight of numbers, which also makes them cocky, and sometimes inefficient. 

The Rebels know this and use it to their advantage, that's for sure, but I don't think that X-wing pilots are as low on the skill ladder as one would think, and TIE pilots are probably not as high, bringing the disparity down. The major factor in the X-wing's success as a multi-role aerospace superiority fighter/interceptor is its survivability. It is generally a tougher frame, but not by a whole lot. The real survivability is it's deflector screens or shields. Hyprdrive gives it the ability to hit and run or hit and fade even without a carrier in it's immediate vicinity, and carrying a number of small torps means it can pack a major punch, and gives it combat capabilities the TIE simply doesn't have. 

Also, I'm fairly sure Rebs have done more than a little training on the Z-95 Headhunter, its pretty much the predecessor for the X-wing. 

Wow... sorry for the seeming rant. Plz don't kill me, HeySeuss.