I've read a number of reviews saying that the 70-300mm IS has far better optics (canon has done two lenses in this range with IS - the first one was a disaster, quality wise, from what i've read - the second is supposed to be far better - but i think i'd want to play with/rent one for a bit first). But yah the 75-300mm IS lens, which was the earlier model ranks somewhere around the 75-300mm that I have at the moment. That said, the turning front element would drive me nuts if i'm trying to shoot outdoors with a circular polarizer on the lens (which I've done a bit)

As for resale value on the offbrand - I tend to buy my offbrand lenses used anyway precisely because of that : )

I've done a little bit of trying to shoot with a long prime, but often times when i'm shooting with a long lens i'm also not in a position to move around much, so I find myself wanting the zoom. (On the shorter/wider end I often have more freedom to move). Still might be worth playing with again.