My Opinion:
M1A SOCOM is a great rifle, but it is no supermatch. Ammunition is such a factor with these guns. I've had M1A scout, M1A National Match, a M1A super match and I only now found the handload combo to get my M1A SM to sub 1/2" @ 100m, (less then 1" was easy). Amazing for a semi .308 and I'm as happy as a pig in mud.
(Don't ask me to post grouping pictures - I could care less if you believe me - course I did shoot 500m attaining .005" groups... in the wind with my brand new $400 dollar savage with an accutrigger.... oh and yeah it was raining uphill)
BULLETS:
I have a AR in 16" and it is fine. The problem with the AR's that I've found is the long range stability of the bullets in wind. An AR firing 55's at 300m is gonna be fine (barrel quality and twist is a huge modifier) if there is no wind. I have found with my HK SL8 shooting 75gr's a huge difference at 300m over the 55's. Most AR series rifles (especially surplus) do not have the twist rate to stabilize the heavier bullets.
IF your purpose is to know which is better for a service rifle match with targets up to 300m it would come down to WHERE you are shooting (in my opinion anyways). If your shooting in a traditionally windy range I would suggest the .308. The heavier 155/168/175 grain bullet will not be blow off zero as much as the 55gr .223 bullet.
If you are going to be shooting game the .308 is better as I'm not sure that the .223 is allowed for game all the provinces.
COST:
If your into accuracy vs cost - the .223 is much cheaper. like half... especially if you handload/reload.
MY CHOICE:
If I had to choose a rifle a standard AR-15 @ 16" or a SOCOM 16" I would go with the SOCOM anyday. Springfield rifles shoot well. Their warranty is awesome - unless you have an ejector failure (if you have PM me and I can help you track down one quick). AR-15's also have the little problem of guck getting in action due to their gas system. The gas system on the SOCOM does not suffer this problem.
OPTICS:
I have tried every mount for the M1A rifles and I've found the SMITH ENTERPRISES to be by far the most stable. The fact that you can't tie the scope mount to the horizontal axis of the rifle but rather only on the stripper clip guide and the side of the action is a definate weakness. the new Troy mount looks like it makes up for this but in my opinion is fugly.
A flap top AR suffers none of these problems. You can get picitanny rail/flat top recievers that mount optics beautifully.
I recently purchased a XCR and it shoots aweseome. the 68gr's come out nice and flat and fly far. (trigger blows goats but they just released news that a match single or two stage will be available).
What it boils down to:
NO matter what bullet/gun combo you have there is no easy answer. I have both; and certain situations call for one or the other. If I only had the choice to buy ONE guy (due to funds) I would pick up a M1A Scout. With the rail directly on the barrel and a high eye relief scope on it, it is a FAWESOME combo and it can shoot - throw a nation match stock on it and you will be a happy camper, it is not as popular as the AR platform but with wind and conditions it is definatley more forgiving.... mind the recoil
I hope my blatherings help you.