It doesn't look like there's much between them Mark. I've got the 70-300 and as is mentioned above, it's a bit unwieldy when fully zoomed, but I'm quite happy with it. The only reason I can think of for going for the 90-300 is that it provides more of an excuse to buy another main lens at some point in the future because if the 18-55 kit lens is the same as the one I got with my 350D, you might find that it's a bit poorly made. Image quality is OK, but physically, I find it a bit duff. It hasn't stood the test of time very well (which could just be cos I don't look after it properly).