Get the one best for you.
1) Do you mainly need 3G only on iPad and not other devices? This would be a huge reason to just choose iPad, more convenient.
2) How is AT&T Versus Verizon in your area?
Honestly, if AT&T & Verizon are BOTH good in your area, only reason to go Verizon would be if you need more devices on the net on a regular basis.