Speaking from personal experience I prefer Unity3D.
I spent a long time trying to make a multiplayer game with UDK, for the same reasons as you state (easier to make weapons, the multiplayer code works well, etc). However I found it to be an uphill battle every step of the way. Their documentation on UDN is far from complete and in many places outdated. Many functions provided you have no real way of knowing exactly how they work or how you are supposed to use them.
I had to spend enormous amounts of times on the forum and for quite a lot of issues I never got any responses (because no-one else knew either).
And it is also pretty easy to see that Epic Games don’t give much support to UDK users either since they rarely try to help, and quite a few bugs found in UDK hadn’t been fixed after more than a year after they had first been reported.
You have to make more stuff from scratch in Unity, but I find it to be a lot faster than when I tried to do the same in UDK, plus I feel comfortable in what i do in Unity, which I never quite managed to do in UDK after the 2 years I spent on using it.
Also UDK has a limit on how many players can connect to the servers, so if you wanted to make an MMO you would have quite a lot of work ahead of you to get rid of that (would probably have to write your own networking DLL). Whereas in Unity there are a couple of networking plugins that boast that they would work excellent for MMO’s, albeit not for free.
You get a lot for free in UDK, but when it comes down to it you don’t really know what happens beneath the surface, and that gave me, personally, many frustrating days.
I would say working in UDK would be hard for making an MMO. UDK mostly consists of First person shooter as of Unity3D can get you any type of game you want to make. Just to let you know that UDK only allows 64 players per server which is really low for an MMO. However, you can configure the structure of the client and server(which could be difficult), to allow a certain number of players. You have to prioritise these 3 tasks: Team, Time and Money. It won’t be cheap nor it will be faster to complete. As for a team, you will need people who could make LOTS of models and not to mention, animate them. To put that short, you will need a modeler and an animator who can produce great quality work. Lastly, UDK 4 could help you in some ways but it doesn’t differ from the normal UDK much. After all, this is your choice but we’re just helping you making the best possible game for players to enjoy so choose wisely. I am posting a link down below to help you understand why it’s harder to make an MMO in UDK:
After tasting Unity, i would not recommend using UDK for anything, not only for MMORPG. In the last resort i would better create my own Unity-like mmorpg game engine, than using UDK.