Personally I find Firefox the faster of the two browsers but I find some web pages seem to load up quicker with IE7 PC Advisor been one of them. Its basically what you prefer, there is nothing to stop you using the two, thats what I do, as I have mentioned "I find some web pages seem to load up quicker with IE7"
I used to use Internet Explorer but then I heard about Firefox. Most pages load quicker (always useful for use dial-uppers) and there are lots more options. Although I don't like the Mac-style layout, when I think about going on the 'net, I think about "firing up the fox" <ahem>.