1-yes
2-yes
3-?
4- all cameras are capable of good IQ as long as you put good glass in front of it. 15 vs 18 MP is not a huge number. Either will give you good results s long as you have good glss. The large number of MP are demanding for good glass. It comes down to the need for video- T2i, need for speed/durability- 50d.
I am a recent 50d owner and now own a t2i.
OK. So I can do a double or triple exposure? Just want to make sure. I'm leaving for the store in a few minutes and probably going to buy the T2i.
Shoot. One more thing. The exposure compensation. +/- 2ev? 3ev? 5ev? NEVERMIND...FOUND THAT INFO - 2ev in 1/3 stop increments. 50D is +/-5 but I shoot mostly in manual anyways. 



