Still early to make final conclusion. I will explain more later after the offical ut3 is out.
In the demo, UT3 felt like an old version and UT2k4 feels newer.
First off, the GUI for the game sux. No more tabs, servers etc.
Way to busy in graphics. When you play the game it feels like a forest.
UT3 appears to be overloaded.
There won't many people switching to UT3.
UT3 won't be game of the year.
New <- does not always mean better.
Sometimes, keeping things simple is better than new.
If this is UT3 than UT2k4 is UT4, because it sure feels like we went 1 step forward and 2 steps back.

So far, if you are a map developer then UT3 is probably for you.
If your a player then you will probably be disappointed.
What now? Don't worry, we are going to give Invasion UT2k4 a complete over haul to make it better than any other (including ut3). Yes, we can incoporate some of the stuff from UT3 into UT2k4. We will just have to re-create the meshes, textures, etc. We like the new UT3 redeemer, hoverboard and the walker. The rest is not that exciting nor worth to spend big money upgrading your pc.
Again, this is based on the UT3 Demo version I tested. Wont finalize anything until I get the actual full game.
Note: something else I noticed, the UT3 Demo had a Login and password to play online. Not sure if players will have to pay, but it sure appears that way. If this is true, they will lose even more players for UT3.