If you've got an AR already and the others aren't what your looking to add on for price it should be a little cheaper to add on a piston upper depending on what you'd want.
Just like any other piston rifle I've used or own there really easy to maintain as the Scar and Masada should be.
My POF runs great on Tetra grease since it doesn't run the same as a DI which floods the receiver with carbon and heat.
The others seem like great choices, but you should save "some" money going the piston AR route if you've got an Ar already and you'll still have all those great AR options and parts at your disposal.
The POF's and LWRC's both use lots of standard AR parts and only a few are proprietary.
On my POF the ones that are like the BCG will last a long time and the gas cylinder is very robust and chromelined for long wear.
The op rod and piston you can buy spares of and anything else is standard.
Hell for a couple hundred bucks you can have a whole spare upper and lower stripped receiver too if you'd want.
POF