Afaik i never said that i am writing c++ for the UDK, perhaps you misunderstood me biglines. UDK is really "just" what Helkster says and uses Uscript. We don't have access to the c++ source code, that would cost 500k$. But the things you have access to are basically the same as in the Source-SDK.
Even though the Source-SDK "speaks" c++, you don't have access to everything. You basically have access to the same things in Source-SDK and UDK. The big difference is that the UDK's structure is much cleaner (surely that's something we could argue all day long about
). To implement MMO-mechanics, especially the big-world-aspect, you will have to overcome several difficulties, no matter whether you are using the Source-SDK or the UDK. Both suffer from nearly the same difficulties. They have been created with "just" a limited number of players and fast paced action in mind. This in complete contrast to our "MMO-ambitions". In the Source-SDK you can overcome this, because you have to write c++ code and have to compile it (this would not be possible or at least it would be much harder if the Source SDK would interpret the code you write). To overcome this odd in UDK you can use UDK's DLLBind-functions.
All in all the project is doable in both, the Source-SDK and the UDK. The big differences between those two are that the UDK is free and the UDK's structure, documentation and code are superior compared to the Source-SDK one's (but again, that's a point one could argue all day long about).