It basically comes down to what you want it to do and in my opinion it comes down to three categories: stealth, loud and proud, and lightweight generalist.
If you want stealth, the mk23 is unrivaled. It's a NBB gas pistol which means that it's incredibly quiet with a suppressor, and they can be tuned to hit ranges well past your MED with the aftermarket parts available. However, if you need to shoot them fast, they have a really squishy trigger which has an awful trigger pull on double action, and the force required to pull the trigger will usually shift your barrel ever so slightly and cause you to miss a lot of shots if shooting it fast.
If you want a bit of firepower and going the loud and proud route, a compact SMG or SBR is great. Lots of people have recommended the Cyma mp5k and I can wholeheartedly agree with them; I have one and while it's a bit heavy, they can shoot phenomenally well. I'm also looking at my boneyard JG skorpion AEP and thinking about getting it fixed up and tuned to be a nasty lightweight BB hose for Vietnam games; make the GIs think twice about charging this sniper!
Otherwise I'd stick with the TM HK45 for a lighter weight alternative to the compact SMG. GBB pistols are great as generalist sniper sidearms as they're very small and easy to carry, and you can still put a good amount of rounds down at anyone who gets within your MED. I frequently use my TM hi-capa when sniping. You could also look at getting a Glock 18 or AAP-01 for a full-auto GBB pistol, but they tend to need durability upgrades if you want to use them full-auto a lot, as well as more efficient parts because it chews through a lot of gas and loses all consistency. Or you could look at a Cyma AEP, but they don't shoot over ~0.5J due to the limitations in design (they're so small and have a tiny gearbox); basically, full-auto on a GBB pistol isn't too practical, and AEPs lack punch to where you need to put a few BBs into people for them to notice them; a 0.2 or 0.25 BB at 0.5J doesn't make much impact.
Personally, I tend to only use the compact SMG at larger or special events like AI500 or Vietnam filmsim games. At regular skirmish days, I don't tend to need the extra firepower and it's a lot of added weight, so I typically bring either a mk23 if I want to be sneaky or my hi-capa if I want a bit more punch.
So, 100% a pistol all the way for me (although using an AAP-01 it's not the quietest).
