The second one is the most appropriate, in my opinion. ^^ Although Sakura is the cutest and best represented in the first one.
I could immediately tell it's her. WAIII!!! I can't choose though! BOTH!!! YES!
The second one as the main drawing - but with Sakura smiling or showing that she likes where she is/being hugged by Nadie - (somewhere were you can see it well) and the first one use it as..well..maybe for a second page on that site.
Oh, have you guys thought about the layout of the site?
I'm thinking about the first(main) page being composed mostly of the second drawing and a nice "Happy Birthday" message (perhaps in japanese too?) and a second page on which all the signatures from everybody are displayed on the background of the frirs drawing.^^
Grr, that might not make a lot of sense to some and might also sound hilariously put, but it's my idea. :P
Later Edit (after seeing Zuiyon's last post): Or the fist page could also contain all the signatures on the left side of the scree, on the blank area in a transparent window or something with scrolling, so the drawing can always be visible. (And yes, Sakura would be looking at the list amazed at all the sigs XD, like Kera-chan said)
Err...the other drawing could, again be on the second page on which all the sites that participated would be listed. XD Ok...this might be even more senseless haha.