I've been using IE 7 for a few weeks now and it works great. In the past I ran firefox because of the tabs and extensions. I've always found IE to be faster than FireFox and this trend continues with IE7. IE's zones are nice. I love the 'no flash' extension for Firefox.
Firefox does have some memory issues. The application uses way too much memory in it's default state and will gobble up more and more memory as you open tabs. Forunately, you can limit the amount of memory that firefox uses via the about
If you use different operating systems Firefox is nice because you'll have a consistent interface.
Opera is good too!
Re: Mac vs PC. Apple's move from OS-9 OS-X was a big improvement for the Mac. At this point these OSes have a lot more in common than they are different. They keep borrowing the good ideas from the other side. They are both good.
I've been using Ubuntu Linux
for awhile and it's outstanding. Ubuntu recognized all my hardware (including a DVD burner, USB hard disk, and FireWire DV capture.) It's free and the install is faster and easier than Mac and Windows. There are thousands of high quality, free, open source programs available via their 'add/remove programs' repositories. It comes with Open Office and FireFox preinstalled. Unfortunately I haven't found a decent audio player, but I'm sure one will come along.