I have been shooting a 6.5 PPC AR15 for almost 3 years so I can provide some input on this.

The 6.5mm Grendel in its final version will be a slightly improved version of the 6.5 PPC.  Lapua is shortening the neck by pushing the shoulder forward slightly to improve durability in a semi-auto rifle and for reloading. Of course, a side benefit of this is an increase in powder capacity.

Alexander Arms has engineered a magazine to work with the PPC based case.  As you mentioned, 10 rounds is the limit for civilians.   However, the 6.8x43 does not use a stock .223 magazine either so if the 6.8RemSPC ever does become a commercial offering, 10 rounds will be the limit too.   Another thing about magazines is thoughts of having a AK magazine work.  The 7.62x39 is a very tapered case where PPC cases have minimal body taper.  AK magazines are designed to work with the very tapered case hence the very radical curve in their design, not a straight case body like the 6.8RemSPC or PPC.

As far as which one? Take a ballistics computer or your trusty reloading manual from Sierra or Nosler and compare a 6.8 mm Hornady 110 grain VMAX with a .370 BC moving at 2500 fps to a 6.5mm 123 grain Lapua Scenar with a .547 BC moving at 2650 fps.

Accuracy wise, I maintain .5 MOA groups out to 500+ yards with my 6.5 PPC.  Based on the shooters skill, maintaining a MOA or sub MOA group to 1,000 yards is possible.   The rifle tested in the article has a longer throat and 1/9 twist.  Mine has a tighter throat and 1/8 twist barrel which I believe the article mentions as a refinement coming in the production version.

Just bought a box of .223 WSSM 55 grain Ballistic Silvertip ammo and paid $20.00 for 20 rounds at a large dealer in Houston.  I have been taking the rounds apart and been making dummy 6.5mm WSSM rounds for another project on my drawing board.   While factory Winchester loadings (with 55 grain or 64 grain bullets) would fit within the magazine length restriction, 80 grain .223, 107 grain 6mm and 123 grain 6.5mm bullets would not fit within their respective cartridges as magazine length loaded rounds.

Another real, possibly big, issue with the WSSM in an AR15 has to do with case pressure which on the WSSM is 65,000 PSI.  While the .223 Rem is 55,000,,there is a large difference in case head area between it and the WSSM.  Take 65,000 PSI or even 55,000 PSI on that large of a case head and bolt thrust becomes a concern.  I hope Olympic has tested their components to the limit.   I would hate to be the one to learn of a weakness with a 65,000 PSI round and my face inches from the receiver.   Don't get me wrong, The WSSM is a cool round and I will be doing a 6.5 WSSM in a single shot bolt gun for f-class competition.  Even then, the 6.5 WSSM is extemely similar to the .260 Remington.

I dont know what the cost of 6.5mm Grendel loaded rounds will be.  However, I would suspect that loaded ammo will be within the same $15-$20 per 20 round range of the WSSM or even .223 Remington with premium bullets and new manufacture cases.  Of course, the .223 Remington has remanufactured ammo and surplus ammo available at a lower costs, but the .223 Remington cannot do what either the Grendel or a WSSM round can do.

Empty Lapua brass for the Grendel will run 30-40 cents per case which is a good price for Lapua quality.

As far as magazines, I was told this past week by a company involved in the 6.8RemSPC project that they will be making special built AR15 magazines for that cartridge.  Whether that involves only a follower or modification to the feed lips as well is not public knowledge. Of course, whether the 6.8RemSPC ever comes to the commercial market is still up in the air and that decision would be up to the US Dept of Defense since it is a DOD project and not a commercial project that the DOD took interest in.  

On the 6.5mm Grendel, Alexander Arms will have a 25 round magazine that is for military and law enforcement customers.

In either case, 10 rounders is all civilians can buy,,,unless of course the 1994 crime bill expires.  If you want to modify your own magazines with followers from 10 round magazines and do other tweaking,,,that is your call.