I have both. Buy a SCAR.
The LWRC is a very good 223, IMO the Scar is a great one. My SCAR is more accurate, lighter, has a folding buttstock that is more Ergonomic, it's currently being used by some of our armed forces. The LWRC may be seeing some action but I don't think near as much as the SCAR has.
The LWRC is a good system but it's still proprietary. My calls to their customer service have yet to result in me being able buy spare parts for it. IMO anything that you disassemble when cleaning the rifle is likely to get lost somehow so it's good to have spares. I haven't checked in the last couple months so maybe it's changed now but now being able to buy spare parts for a proprietary system is a big negative against the LWRC.
The SCAR does have a reciprocating bolt handle. It never bothered me on AK's,. PSL's, or Sig 556's but some people will find that they don't like it if they leave their thumbs in the wrong place. I put mine on the same side as it is on my AK's, PSL's and SIGs to maintain the same manual of arms and protect my left thumb.
It's just my opinion so take it for what it's worth. I don't think that piston systems are that necessary for a semi-auto AR. They probably come into thier own on full auto guns but the extra weight, expense and proprietary parts make it less of a benefit on a Semi-Auto. I think that DI's may be slightly more accurate though not really enough to matter on an M-4 type.
So in conclusion, neither is a bad choice the LWRC is a good gun with a few drawbacks - price and parts but in my mind the SCAR is a clearly better gun. I'f I was headed to a gunfight I'd pick the SCAR over all of my other 223's......................................................................DJ