Nothing wrong with having 2 pistols, but you'll find out real quick why nobody else runs 2 pistols, it's far more of a hassle than it seems at first thought. Keep in mind you have to reload them at some point lol
Use propane in the summer, when it gets down below 10 degrees and your gas efficiency really starts suffering, you can run MAPP gas (yellow cans at cad tire/home depot). It's an extra 15psi or so, but more stable in the cold. You can run MAPP in the summer, but keep in mind the mags are barely designed to hold propane, and they don't have pressure relief valves.
So if you use MAPP/propane in -5, make sure to vent most of the gas out before you store the mags at room temp or you'll over-pressurize them and create leaks.
Personally, I'm a huge fan of plastic slides. The less weight you have on the slide, the higher your gas efficiency will be. For example; usp compact with a metal slide can do about 35-40 shots before running out of gas. usp compact with a plastic slide can do 80-90.
Not so practical in summer, but REALLY makes a big difference in winter.
Advantage to the HiCapa is the dual stack mags, and you can get split slides where only the back half cycles. The less weight on the slide, the better.
If you get an AEG, get something short to suite your style and environment.
P90 would be naturally great, but unfortunately the trigger pull on them is nightmarishly long and super squishy.
Probably something like an MP5? You can set them up to shoot pie plates at 250ft pretty easily, and the mags are tiny and weigh nothing.