Hmm, well I run both, and this is my 2 cents.
I have a CMMG conversion kit, so I've been able to compare both systems on the same rifle.
I will note, that if you're going to run a conversion, timing becomes somewhat more complicated and there are more things to consider for proper operation then there is with DI.
Bolt lug round off seemed to be more a problem for me then carrier tilt.
If you are buying a ready made Piston rifle like the Core15, then all those details, I would assume, should be taken care of for you, and you do not have to worry about them.
I've run the piston with both 11.5" and 16" barrels.
On the 16", it ate everything I've fed it.
One the 11.5" the lack of back pressure has made it more temperamental, preferring gassier or hotter loads to cycle.
I reload, so this is something I think about. If you are shooting factory, you need not concern yourself with this, as even the weakest runt ammo, like MFS steel, cycled the 11.5" piston just fine.
Weight, indeed the piston system made my rifle more "top heavy". On paper it doesn't seem like much, but in practice, you can feel it. A DI AR with a standard profile barrel has barely any weight past the front of the receiver, making it feel lighter then it is. Once you add the weight of a piston system, it changes the balance and can make it feel a lot more heavier then the DI then it actually is.
Cleanliness:
What you have to consider here, is what are you shooting and how much ?
If you're going to shoot dirty crap ammo, then it almost completely circumvents the clean running effect of piston.
I put 100 rounds of MFS steel thru' my 11.5" Piston Carbine, the stuff is so dirty, the blowback alone soiled the receiver as if I had fired 100 rounds of quality ammo thru' a DI rifle.
I still had to clean it out. On the other hand, if it is indeed that dirty, imagine how much worse it'd be with DI. I've never fired 100 rounds of MFS thru' a DI carbine so I have no idea.
Another thing to consider is, how often do you clean it.
Indeed, the piston system run cleaner in the receiver, but the piston system itself gets soiled adding another step to your rifles cleaning regiment.
In a DI system I clean the barrel and chamber, take the BCG down, and clean it, wipe out the receiver, re lube, and I'm done.
In the Piston system, it gets the same respect, I clean the barrel and chamber, take the BCG down and clean it, wipe out the receiver, re lube, but I'm not done, I still have to pull the piston out, clean it, clean out the piston tube, and the piston bolt.
So, both systems get the same attention, because remember, your receiver, and bolt is still going to get dirty from blowback, and brass shavings, so it will still need cleaning at some point. It also may get cleaned simply to re apply fresh lube.
So with the DI system you are pulling more fouling our of certain areas, and with the Piston you pull that fouling out of other areas, but in the end, you've added an extra cleaning step with the piston system. My piston gun actually takes me LONGER to clean then the DI system.
Furthermore, I clean after every range session. My Rifles Always go back into my safe, clean, with fresh oil, and ready to fire.
My range session typically are only about 100 rounds.
I find that DI fouling after 100 rounds of my clean burning reloads really isn't that big a deal. I can remove most fouling in short order and I don't have the extra step cleaning the piston.
What benefits you see as far as cleaning go, really come down to what kind of ammo you shoot, and how much you shoot between cleanings.
For the maintnence murphy target shooter, I could see a good piston system being as long faring as the Ol' Dirty Fal, but if you show your high end rifle the respect it deserves as I do, then a piston adds more work only to be balanced by the volume you shoot.
PARTS: The ace card of the debate.
I like to throw this argument completely out the window, for two reasons.
One, because I'm a casual target shooter with multiple AR systems.
Two, because I run a conversion system.
As a casual target shooter, which many of us here are, if my Piston gun goes down, I simply bring a different gun to the range, while I figure out what I need to do to fix the piston.
Fussing over parts is silly in this context.
The argument of parts only holds water if you are an Operator Operating in Operations, of a Competition shooter. Sure, I can't just run down to the local shop and pick up piston parts, but, if I take my gun that seriously, I can plan ahead.
And if you think about it, most of us can't get DI parts that quick anyway cause we mostly have to order online, the same as the Piston parts. For that reason, most AR owners have spare parts on hand. Treat your piston gun the same way, and there is no problem.
As a person who runs a Conversion Piston, a DI system IS my spare parts. If my piston goes down, I un bolt the piston assy', and bolt on a DI assy'. Easy peasy. I can do that in maybe 10 - 15 mins, and I won't miss out on any range time.
I will also point out, that one of the whole points of getting a piston system, is for the reliability to the effect that they are mush less likely to fail in such a way that you are in the need for any proprietary parts. The most likely parts to fail, like the extractor, are interchangeable with DI parts. And many Piston systems even use the same bolt.
When you think about it, there is not much that can go wrong with a piston. Not the system I have anyway.
And finally, accuracy:
What are you using this for ? To shoot groups ?
My piston is strapped to an 11.5" Carbine with a standard style handguard (as in not free floated). Accuracy is a moot point, as I would not be able to tell the difference in practice.
In theory, yes, a piston cannot be as accurate, as it can greater augment the barrels dynamics. For that reason, I can't think of any finely tuned OBR off the top of my head that runs a thumper.
Unless you are trying to shoot tight precision groups off a bench rest, I would not worry about this. A typical AR carbine is likely to give you 2moa. If it's giving you 3moa, it's doing what it's designed to do, and is still going to be more accurate then you are standing. Running a piston isn't going to mysteriously open groups up to minute of barn.
When I got my first AR, I bought into piston cause it's what I knew running other Semi Rifles for years. I'm slowly growing to love the DI system, but am still on the fence.
I will say, I'm in less of a rush to convert my DI's to Piston then I am to convert my Piston to DI, and the only real reason I'd move away from it, it due to the weight, and the fact that it will not cycle my most economical reloads.
Seems my 2 cents turned into a couple of bucks. Takes it for what it's worth, it's just my perspective having an intimate knowledge of both systems.
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