The MOE handguards are quite solid and very comfortable. I have a carbine-length (the SL uses a mid-length) one on my bushmaster with a long rail section and a VFG2 underneath and a MOE Scout Light and MSA on the support (left for me) side. This is a very lightweight set-up.
The rails on the ARM-R handguard are very nice, but I dare say if your mission doesn't require you to rapidly change out a wide range of accessories on a moments notice, they aren't really necessary anymore. I'd bet that's a big part of why the company has made such improvements as the REPR and SPR rails and the entirely new IC upper. With the ability to add rail sections only where you need them, having them all over is just dead weight, IMHO.
If it were me, I'd get the M6SL and just put MOE stuff where I need it. I've said before that I don't care for the MS2 sling, but that MSA makes a darn fine "two point" out of it. Also, the MOE scout light mount has a 1913 rail on it making it perfect for a wide range of lights, not just the Scout Light which bolts right to it.
If you don't want the "A-frame" FSB up front, look at the SPR. That would definitely be worth the extra coin.