Marco’s got a very comprehensive FAQ on all the decisions you’ll need to make when considering the iPad 3. I couldn’t have said it any better myself.
This quote really cemented my decision to go for a 4G enabled model:
I used my iPad 2 with tethering for a few months. It wasn’t nearly as good as giving the iPad its own 3G service. With tethering, the iPad isn’t always connected. Getting and keeping it connected takes a bit of effort. Sure, it’s not a lot, but that friction adds up and just made me use it a lot less.