There is a lot of difference bro... there's 1 and 1/3 stops more light coming in from the f1.8 compared to the f2.8 if i am not wrong.

that said, I have used the 16-35L on crop sensor before, doesn't really give me the wideness I am looking for, yes, it is still wide on a crop sensor, but only 25.6mm (narrower than what you can get with 24-105 on FF). so if you are looking for true wide angle, you should get the 10-22mm.

well... if he can afford that, i am sure there's no need for such topic