Black rain rifles are high quality and nice weapons. You'll be happy with them.
The piston driven AR is somewhat different that normal "direct impingement" AR's Its all about gas and where it is bled off. On a piston driven AR, the gas is bled off through a piston at the front of the weapon, on top of the barrel. That piston can be adjusted for how much gas is released. They are cleaner running and a smoother shooting rifle in my opinion.
The DI rifles bleed the gas off at the bolt. Not a bad system at all, but much dirtier and harder to clean. Normally without an adjustable gas block, the gas cannot be adjusted like a piston driven AR.
I have both, but prefer a piston driven for a couple reasons. In 3 gun I can tune my load to the piston and bleed off more gas. This reduces recoil and allows for quicker follow up shots. The piston driven set up also allows for easier cleaning and less mess. The DI guns make for a lot of cleaning. The piston bolt and carrier can usually be brushed off and wiped clean, whereas a DI gun bolt needs scrubbed and sometimes, scraped. The bolt carrier is normally okay though and can be cleaned easily. Alot has to do with how often you clean and how well you maintain your weapon.
I prefer the piston driven AR's and don't mind the extra cost to get that system.
You won't be disappointed either way and either will treat you well.
