I have some questions about unity (free).
What is its ease of use compared to UDK?
What are the graphics of the engine compared to UDK?
Which would be more suitible for a MULTIPLAYER WW1 game?
Well, I’ll start with ease of use.
Winner: UNITY, a hundred times over. Many features, particularly UnrealScript, are all but undocumented, and what documentation is there is already outdated.
Not only is UDK iterating faster than its documentation, but they’re removing features as they go. This last release removed support for Pixel Shader 3… a bit of a blow if you want to make games for anyone who doesn’t have a gaming computer.
Although, to give UDK credit, a lot of the map editor features, particularly Kismet (a graphical scripting engine) and Matinee (an animation system), were very powerful and easy to use once I watched a video tutorial on them.
Graphics:
UDK, definitely UDK, although Unity is catching up with 3.
Keep in mind for good graphics on Unity, you have to buy Unity Pro. The Free version, at least of 2.6, does not allow dynamic shadows besides blob shadows and does not allow anything remotely resembling a render-to-texture effect, including post-process filters.
Multiplayer games:
Are you making an FPS? Probably UDK. Otherwise, Unity’s pretty good, although I’ve just barely started to mess with its networking capabilities.
Scripting is one of hose things you will need to do. In udk, you close to close the editor, open the file, recompile in frontend, and start udk. But in unity, you can do all of that without leaving th editor. I also think that this comunity is better.
As for graphics, udk win. For now. It has a built in lightmapper that simulates ligh photons. But once unity 3 comes out, they might be even! I would stick around and make some props before starting on the engine.
As for the multiplayer, unity is better. You can play your muliplayer game on different machines. One guy can be on facebook, one on your website, one on a pc, and one on a mac. And they will be able to play together. Plus udk doesn’t have ipad, iPhone, wii, xbox, ps3, and mac support.
There’s plenty of threads about this already:
http://forum.unity3d.com/viewtopic.php?t=53567
http://forum.unity3d.com/viewtopic.php?t=36040
But please feel free to rehash it all over again 
oh sorry didnt see those posts 
And will there be a free version of unity 3? If yes, I’ll go unity- if no, i’ll go UDK.
:?
Yes, there will be a free version of Unity 3.
I’ll just say also, after looking at many other options, Unity’s Editor extensibility - the fact that you can roll your own tools and integrate them in the IDE is absolutely fantastic, and like no other editor out there.
It’s easy to underestimate this point until you need it 
At this time:
for a serious game (AAA class) you definitely need UDK over Unity, it has the most features out of the box, it’s the fastest renderer of the two, and mature (tons of games have proven it).
In the future:
Might be on par, IF and ONLY IF revision 3 of Unity is pimped quite a lot, as in render speed, lightning, AI, Editor features build in, FBX support, third party tools support, either build-in or available (e.g. Speedtree, FaceFX, Scaleform FX).
Furthermore, the Indie version MUST have more features under it’s belt. If not, the Unity is LOST on the long run, for hobbiests.
Unity 3D will never be lost.
It will always be kept alive by its developers and the great community,who are constantly making new things for Unity 3D.
Neither will year-old threads, with you around.