Go both! LOL! Seriously there will alway be something coming and something closed for rehab. Something old and something new. So I guess it all depends on how important the new additions to the parks are. In 2010 we will get World of Color and the beginnings of some rather dramatic changes in the atmosphere and architecture of California Adventure. In 2011 we are getting the new Little Mermaid and a revamped Star Tours. In 2012 comes Cars land and the finishing touches of the DCA revamp including the Red Trolly system. I'm guessing in 2013 that we may see a new E-ticket at DL if you can believe Al's updates (Which are generally pretty accurate).
Disneyland is, in my opinion, the most fantastic single theme park in the country right NOW. It beats any single park in WDW, although I love WDW for the variety of things to do and places to go. The DL Resort of going to have something new every year for the next 5 years at least. There will alway be a reason to wait.
If I HAD to choose, I would go after World of Color opens (this year). You can ALWAYS go back in a few more years after more stuff changes.
So I say.........go go go in 2010!