What you want is a pistol that suits your hand, usually that means trying a bunch out. You might think you're a USP guy but will suddenly realize a Glock feels right.
Then you want to make sure you can get spare magazines so it should be from a well known brand. After that you need a holster so weird stuff like Desert Eagles can be a mistake.
Green vs. Co2, pretty much any pistol that's decent for a skirmish will be green. Get an adapter and run propane with self mixed silicon oil to save cash.
For your specific models mentioned there isn't much choice in the P99 world. You're pretty well stuck with a buggy Maruzen. For USPs there's good choice from KSC/KWA and a coming soon flavor from TM. There are less common USP models from Tanaka and Tanio Koba too, along with the NBB MK23 from TM.
If I was looking for a USP right now I think it would most likely be a KWA USP Tactical that I would home in on.
Airsoft, where nothing is hurt but feelings.