iPad 1 this evening after work (no HD or dual core)
The iPad 1 and 2 differences are negligible unless you use video chat.
I have a multitude of tablet, phone and ebook devices for development purposes, far more than what's in the picture I posted. ( though it was deemed not economically viable to even test on W7P )
They're very ambiguous, so unless you're specifically looking for e-ink, a scroll wheel for games, or another specific hardware attribute, you should probably go for the cheapest thing with a 1Ghz processor.
The best use of an iPad for developers is checking off a box that says "works on iPad" as you pass off to QA.
Otherwise an executive's son somewhere half way around the world will tell his ignorant and rich daddy that you are teh bad developer, and how you should be put into a basement dungeon at the corporation like Milton Waddams from Office Space.