If you intend to use it as a sniper secondary then Tokyo Marui hands down. It is the quietest and most accurate handgun. The LAM usint is quite useless and the magazines can't handle green gas because of a valve that releases the pressure.
STTI (KJW) makes a very good clone of it, but the silencer is plastic and has an inner barrel that eliminates the silencing and raises the fps above the limit. So it must be gutted but that's not that easy because it's glued together quite hard. But if you can do that (or you get a silencer with foam and a 16+ thread), it does work very good and it's much cheaper than the TM. This one's magazine can handle green gas.
If you don't want it beacause of the quietness, then using an MK23 is not very practical because it's uncomfortably big. And because these are NBBs the trigger pull is extreme long. I hate that on these.