I've been shooting M1A's in various configurations most of my adult life and I'm fed up with the "scope mount of the week" game.
I'm looking at various modles of AR 308's and the choices are pretty overwhelming.
I would like to stay with something that uses M1A mags, I have plenty of GI's. But that is not set in concrete.
I want the most accurate and reliable flat top 308 AR.
Would like the free floated 4 rail system.

Thanks in advance.

If you're wanting accuracy, railed FF tube, uses M1A mags, and flat top, you just EXACTLY described an Armalite AR10T.  They're the only game in town that uses your mags and has the other stuff you desire readily available.
FAL (and clones) are gonna have the same scope mounting issues as your M1A.  HK makes a highly accurate rifle (PSG or SR9) but the $4K-10K pricetag makes it little more than a pipe dream for me.  Knight makes the SR25, but you're looking at around $3K and between 3 hard to find mags and a KAC railed tube, you're adding close to another grand to the price.
I have a DPMS 308, both the 24" and a 16".  I love mine, but accuracy and flat top are the only parts that completely fit your criteria. It uses the same mags as the SR25--nearly impossible to find for less than $125, and the only FF railed tube available is the fine product SniperX is selling on the EE

I have to agree with Colt45.  If you get their (ArmaLite's) 20 rd mags or mag conversion kits now they come with a plate in back that lets them be used in both the M1A and AR-10.  You can switch out followers so that the bolt will stop on one or the other.  I think that if the AWB dies they won't need to put that plate on the back so the interchaneability might be lost.  But then again, mags will be easier to get for both and the 14 mags can be used for the 10 if the law changes again later.

I have an early SR25, 3 AR-10s, a Bushmaster 308 and a DPMS.  The SR and AR are best for quality.  I give the edge to ArmaLite because of higher quality features like forged receivers.  The SR is polished but I think the guy who tells why they have to do it (to match up mismatched receivers) is right.

The DPMS and the Fulton (to be made from DPMS parts) is a cheaper rifle.  The DPMS can shoot just great but really doesn't look good.  The pictures of the Fulton show the same thing.  If you can't afford a higher grade rifle they're OK, but you might consider one of the FALs if you want to pay a bit less and are willing to take advantage of the cost savings from the surplus parts kits used.

The Bushmaster is the funny one.  High price but inexpensively made.  The receivers seem to be die castings and won't even be as strong as the extrusions used on the SR, DPMS, and Fulton rifles, much less the forgings of the ArmaLite.  By all rights the Bushy should be the cheapest of them all.  They've got cute ambi mag catch and bolt release.  The Police and military friends don't care for that because they'd rather have identical controls so they can switch back and forth with ARs without getting confused at a touchy time.

Staying with the M1A/M14 based mags seems to allow a stronger rifle too.  The barrel extension of the rifles made for the FAL mags have to be weakend by cutting a feed ramp, or the mag has to be pretty nastily modified.  That's supposedly one of the reasons that ArmaLite didn't use the FAL mag.

Word is that prices of the AR-10 mags will drop to under $40 if they can make them new.