Having upgraded to iPad 2 just a few days ago I would definitely go for the 2nd gen.
For me the most important improvement is RAM size. Switching between apps makes a huge difference.
Also when browsing the web, you don't have to wait for the page to render when scrolling fast (there's almost no "checkerboard" image).
Talking about the comfort of use and responsiveness, I would say it is like the difference between iPhone 3g and iPhone 4.