We’re using UDK now, allthough we started on source our former programmer let us down.
Without a good C++ Coder it won’t be possible to make this kind of mod on source.
Also the tools are somewhat outdated and even if valve will update them anytime soon, i’m sure they’ll fuck up the whole SDK a few times again.
UnrealScript is also very powerfull, the UDK is much more advanced in terms of handling the assets and the model based mapping is much closer to how Retro did the Prime games.
Oh and you’re able to release it as stand alone and don’t have to own a game which is using the engine just to play a mod based on it.
As for Unity i don’t know much about it, but if you wanna make a full blown game for several plattforms, it would be better to choose something not based on a franchise of a big company such as Nintendo. Just in case it turns out pretty well and you wanna sell the game to make money.
PV is a good example, they once were a Metroid mod but Nintendo told them to stop using their IP.
I guess they just developed the mod at the wrong time. (Prime 2&3 still in production, MP: Hunters on the DS)
Anyway, there’s no Metroid content left in PV but it’s still heavily inspired by it.