I suspect that you mean the 70-300 IS unless I've missed the 75-300 being IS'd.
I can't quote the relative merits of each but the 70-300IS is about the same price as a 70-200/4 L
Okay, it's not got the same reach but it's way ahead in IQ and will surely crop down with equal results for the occasions where you went over the 200 mark.
This might be the way to go forwards.....
Bob