..and UDK has much higher system requirements. but it depends on your pc, and on what you expect from your potential users... and as I heard UDK is not an easy to use system.

what I think about fps games:
- an ordinary fps game is basically an easy and boring thing to script, but if it can be played online against human players it becomes real fun, but that requires good marketing (=money investment) to achieve high enough number of players, and a nice server background...
- the other good case if the game environment is unique, like that 3DGS project with strange gravity features (I don't remember the name). In this case 3DGS is really the best choice due to its fast scripting language (and in this case gameplay is more important than shiny graphics).
- the 3rd case if it has a popular story like Star Wars, but usually they are badly not royalty free... in this case graphics has to satisfy the highest expectations, to approximate the original story environment.

for making online games Esenthel has some ready set of MMO scripts, both for client and server side (what I have not tested yet), and has also a server support for licensed users. building up game levels and making nice graphics can be done faster than in 3DGS without licence limitations. I think in this case it is a good choice. but when you need to make a lot of own scripting, you have to be a skilled programmer to expand Esenthel's functionalities.