The American Stratocasters are made of higher quality parts, wood,... and therefor they are better instruments. The have better pickups (although still not good to my liking, and single coils have a low output not high as mentioned before), better hardware such as tuning keys, bridge. The finish of the neck is also better I think.
Then again. The difference between an american and mexican model is not as big anymore as it used to be. On paper the mayority of the materials used is the same. But consistency of the quality of the instruments is much higher of the American models. You can say that, every american strat is the same. Mexican strats are different, you can find very good ones that are almost like an american and you have really bad ones.
Also only some of the American models have 22 frets (which is a must for me). The other guitars have 21 frets (like the original stratocasters but a number which I find pretty limiting). The wiring is different so the controls work a bit different on both models.
Regarding the sound I have to agree with the previous poster. You probaby won't hear a difference. But then again, I think that the mayor factor in your overall sound is your amp and not your guitar. But certainly the feel of those expensive guitars is much, much better than the feel of the cheaper models
And a final remark, the American models are certainly not overpriced. These days you see quite a lot of kids who play on their chinese (Squier) or mexican strats, consider themselves to be guitar experts, who claim the american models are totally not better than the mexican.
An immortal unbounded by fate.
...and amidst the clouds of the melodically similiar
a rising force rips through
with incredible asunder
the cross of chains on his left
and timeless gold on his right;
a Fender in both... faster than the speed of light.