I'm running an 8 core amd (see specs below) and I'm quite happy with it. It's a real budget machine with some punch. I haven't owned an I7, so can't really compare, but I have more money in my wallet.
[edit] I should have added the I7's are usually faster, so it depends a lot on your budget. Also, I think you can generally go with a cheaper motherboard with AMD, but don't hold me to that -- I've never checked that out.
I did purchase a sealed water cooling unit so I could actually do renders when summer hits.