If they enable the WiFi hotspot, then I would go this route for my 3G iPad. The advantage I see are:
- iPad has 2GB data for $25, this would only cost me $20
- I could share the total 4GB between iPhone and iPad (though my iPhone is currently unlimited, I never come close to 0.5GB usage).
- My iPad does not benefit from my company's employee discount agreement with AT&T, but the tethering option would (so closer to $16.50 instead of $20).
The disadvantages I see are:
- Switching off unlimited would rule out video streaming over 3G if I was ever so inclined.
- My iPhone's battery does not compare with my iPad's battery, so heavy iPad usage would drain my iPhone.