The GoogleMe card says: You don't really need my regular card. Just Google me and you can then choose whatever way you want to communicate. The Twitter card says: Let's communicate via Twitter (and email if you must). Making your email address much smaller than your twitter name non-verbally communicates your preference but has your email in case the other person isn't on Twitter.
I had a little bit to try them out now and my GoogleMe card has been the default. It's the Moo.com mini-card product so it's even more different because it's way smaller than a normal card. In the case where I know someone is on Twitter, I'll just hand them the Twitter card. I can tell how tech savy someone is by their reaction when they take them. They either stare down at them in confusion (in which case I hand them the regular card) or they say "hey cool!" and my card just became a piece of unique marketing followed by "Where did you get these?"
Traditional cards are still great, but with social networking becoming more popular than email, (see this article, and this one) it's time to start expanding beyond the traditional.

