to choose I'd go to ZCon. I bet Nissan will have some pretty good events planned seeing as how they're right there.
ZNats was fun when I went in 2007 and it was a very well run event so it's no knock on them. Just saying if I had to choose for 2010 it would be ZCon.
I won't be at either of them though because I'll have a newborn and I'm sure I'd get torn a new one if I went to a Z event in the baby's first month and left mommy at home alone.