One extra consideration - if you only plan on using it occasionally, the built-in 3G may not be much use to you.
The reason is that the iPad takes a micro-SIM, which is still (as far as I know) only used in iPhones and iPads - which means unless you already have an iPhone then you'll need a dedicated 3G Broadband SIM for your iPad, and there aren't any tarrifs suited to occasional use. The micro-SIM can aso be difficult to get hold of when you're abroad, if you wanted "local" data prices rather than roaming.
I have a MiFi, and very rarely use it, but when i do i just swap the SIM out of my phone. Bit of a faff having another thing to carry, but its the tarrif prices that made the difference to me, not the hardware.
David