The cellular ones have real GPS, the WiFi ones don't. If you plan to do much with maps/location on your iPad that might be a factor for you. Not really compelling if you have an iPhone.
For me, 200 MB/month free for life from T-Mobile is a compelling enough argument to spend the extra money. 99% of the time, I won't need or use it; 1% of the time it will save my bacon, and I won't have to use up part of my iPhone data to get the iPad online.
I wish my Mac had a cellular radio too. It is luxurious to be able to call up the entire internet anywhere T-Mobile has bars.