AT&T.
Verizon offered fewer dropped calls compared to AT&T in back to back iPhone 4 tests, but you're not using voice 3G service on an iPad.
AT&T consistently outperformed Verizon when it came to data speed. Both are arguably close in overall coverage, however, Verizon has a larger 3G network, though half AT&T's speed.
Having GSM worldwide compatibility is a plus for traveling outside the US, or as already mentioned, may offer a future buyer more options, therefore may hold a higher resale value compared to a non-GSM model.
The AT&T model would be my choice if it were me, however, I'd also check into unlimited service from Clear if you've got coverage in your area. That's what I'm using now, and I love it.
Wireless Internet — Mobile 4G Wireless Internet Access from CLEAR