Why would you get the WiFi only version if you can have both 3G and WiFi? Not to mention the 3G version has an A-GPS is worth at least $130 in terms of retail pricing. And you don't even have to have 3G turned on. I personally have the 3G version and I carry a WiFi hotspot around instead of subscribing to 3G on the iPad.
And you can't really use the iPhone SIM in the iPad without AT&T blocking your account.