I've been very happy with my 70-300IS. My understanding is that the 75-300 is a far inferior lens.

These were with the 70-300:
http://www.schrock.org/photos/2009-a...G_7009.jpg.php
http://www.schrock.org/photos/2009-a...G_7331.jpg.php
http://www.schrock.org/photos/2008-r.../at6-2.jpg.php

Yeah, it doesn't have the widest aperature, so it's a bit slow, but it also costs a lot less than the 70-200/4 IS, which is still not going to get you that range of 200-300 anyways.

I'm far more likely to upgrade my shorter lens first because I don't really have anything there I'm completely happy with, but at least for me, the 70-300IS has been good enough.

(Note, I make no claims at being any good at all, and I'm not making money from my photos. But I'd say of the pictures I've taken over the years, of the ones that I look at that I'm actually truly proud of, a large percentage of them were taken with the 70-300IS.)