You know, in the last week or two I think I've changed my mind a fair bit on this!
As a DM I really like the ease of prep for 4e, although tracking conditions/marks/etc actually seems to make running 4e a lot more fiddly for me. Unfortunately, I really don't like a lot of the 4e fluff and I've found the 4e adventures thus far to be really terrible. As I don't have the time to create or convert my own stuff that means running 3.5 adventures from Paizo.
Having said that, 4e Eberron has almost changed my mind in the last couple of weeks. My right brain loves the Eberron world, and I think 4e is really the perfect system for running it. That and the fact that the 4e Eberron adventures actually don't seem to be so terrible has me changing my mind to considering running a 4e Eberron game as a DM.
As a player, I've been playing 4e lately, and other than high paragon and epic play I've really enjoyed it. Making up characters using the character builder is a blast, and the mechanics run really fast and smooth as a player (because you only need to keep track of your own conditions/marks/etc).
Having said that, my 3.5 campaign ended in a TPK on Sunday, and someone else is going to run a Pathfinder Legacy of Fire campaign. I've been making up my character and it really reminded me how much more fun and customisation there is when making up a 3.x character.
Maybe it's because of the character builder or 4e having more limited options, but when I make up a 4e character the process seems to be to choose a race/class and then come up with fluff and/or a schtick that matches. When I make up a 3.x character I choose the fluff/schtick first, and then there are so many crunchy options I can choose the right combination of race/class/feat to match pretty much any concept.
So as a DM (right brain?) - 4e for prep and Eberron fluff/adventures, 3e for running and other fluff/adventures.
And as a player (left brain?) - 3e for character creation/advancement, 4e for playing.