Sometimes I forget how few true high ROF setups there seem to be in AZ so I didn't comment sooner.
Most people do high speed M4's because of part availability. Box/Drum mags are far easier to find for the M4 than pretty much any other platform. Same goes for replacement parts. Plus, it's by far the most common gearbox out there, so there's simply more info on high speed M4 setups. That said, you run into the same wall with pretty much any AEG. The ability of the mag to feed fast enough to keep up. I don't have any factual data to provide you on this, but I see no reason why an M4 mag would feed any faster than a similar G36/AK/Whatever mag. A sector clip helps a bit, but you are still going to run into the wall sooner rather than later. There's two ways around that rule.
1. Custom modified box/drum mag. You can find more specific info on ASM but basically you modify a basic electric box mag to run a higher voltage and thereby increase the speed in which the mag feeds. However that's not without issue because if you box mag fails for any reason (and probably will, once again sooner rather than later) or if you are playing at a game that won't allow box mags on M4's then you are SOL.
2. Use a P90. There's multiple reasons for this. First off, the real P90 has a relatively high ROF for it's a bit more appropriate as far as realism goes. Secondly, the P90 mechbox is one of TM's newer designs and is simply a smoother running gearbox. The third and most important issue however is feeding the gun. The P90 has two distinct advantages in this regard. First off, the highcaps are pre-round so you aren't constantly trying to wind the mag while firing. Secondly, you have the advantage of physics. Because the magazines are horizontal and pushing the BB's downwards rather than upwards, the P90 magazines are simply capable of feeding much faster (the standards more than the highcaps) than magazines on more conventional weapons.
With that said, if you have both platforms and are going to be buying more parts anyways, then I see no reason to choose one of your guns over the other beyond personal preference. I would however recommend not using the G&P simply because you'll want to have a nice reliable backup as even in best case scenarios, you are going to experience failures more frequently with a high ROF gun.
In regards to the parts failures.... By extra pistons in advance. Be prepared to blow through a few in a hurry as you tweak the gun. I've never adjusted the AOE on my guns which leads me to believe needing to do so is either baloney or only applies to buying cheap guns/parts. This is especially prevalent on high ROF setups. A part fails on a gun and people blame that part for the failure which may not actually be the case.
