I was musing about this, and tend to break it up into two separate questions...
AR vs Garand design, and .223 vs. .308 cartridge.
So it is really a choice between four guns/types:
- AR-15 in .223
AR-10 in .308
Ruger Mini-14 in .223
M1A in .308
I've been fortunate enough to handle all four of those, though never in an 'occupational' mode.
For the .308 cartridge, unless one goes with a Kel-Tek bullpup or other oddball, it's a choice between the M1A and AR-10, and to me, both felt BIG and HEAVY, but the M1A just felt more like a gun 'should' feel to me, with no stuff sticking out where it didn't need to. (
Of course that could be because the AR-10 was scoped, plus the 'pistol' grip.) I didn't shoot either enough (
and may not be a good enough shot) to tell any accuracy difference, and didn't have a stable of other 'uppers' to swap out so I could shoot .243 or .358 or any of the other fun biggies, either, so the AR-10 didn't have any particular perceived 'advantage'.
Incidentally the Garand is even 'slimmer' due to the lack of protruding magazine, though the front wood is bulkier. Eight shots and storable-loaded 'clips' make for plenty of home-owner firepower, but all the little fragile-looking parts pushing the cartridges up make me nervous (
though never heard of any of them breaking).
As far as the .223-sized guns, it seems like the lack of streamlining with the AR-15 vs. the Mini-14 is much less bothersome, and it also seems like with the wimpier round it would be more important to be more precise in shot placement when able, so any accuracy difference is magnified. Plus, I was a good enough shot to clearly see the difference in accuracy between the platforms (
unlike the larger guns I compared, both the Mini-14 and AR-15 were scoped). I still just 'liked' the feel of the Mini-14 (
it even had a genuine 'hardwood' stock 
) better, but the accuracy of the AR-15 was almost enough to win me over by itself. PLUS, the guys I was shooting with had 'uppers' ranging from 16" shorty to 24" varminter types, scoped and unscoped, that they could change out in a minute. If that same range session had happened now, no doubt someone there would have had a .204 Ruger upper, or a .300 Blackout or .50 Beowulf one, as well.
So.....
My preference would be an "AR" in the short-action cartridges, and an "M" in the long-action cartridges. AR-15 and M1A.
Funny thing is I have a friend who loves his AR-10 (and has a .243 upper for it as well), but prefers his bedded-and-accurized Mini-14 to an AR-15. I think he figures the .223 is mostly for 'plinking' but the .308 is his really 'serious' platform.
As far as cartridge choice.....that depends on what range and power you need, what weight gun/ammo you are willing to carry, and what other firearms you might have in those chamberings.
Dreaming.....
Of course some guys opt for the 7.62x39 cartridge, as more powerful than the .223, yet light weight, but the guns are all either pretty marginal vs. accuracy, or are modifications of AR-15's or Mini-14's that aren't ideal.
What I would like, and would almost be willing to make my "one-gun battery" would be the best of both....
- The 'Garand' design - simple, durable, few parts
a .30 caliber round close to the 7.62x39 in power
a lighter weight firearm that still feels 'like a gun should'
good accuracy - minute of angle or close would be great
suppressable in an effective loading
So,
wouldn't it be nice if Ruger would make a Mini-14 with a heavier barrel, in .300 AAC Blackout...??? 