Both systems are iffy and much depends on your use for the rifle in question. When I worked Product Engineering for the Army Small Cal Weapons Lab, Picatinny Arsenal I had engineering responsibility for the Garand and the M14 and all other rifles, shotguns and SMGs in the Army inventory except the M16 and I did malfunction investigations on them when they came in blown up. As well I had all the engineering files for both systems within fifty feet of my desk. I knew Al Cole and Julio Savioli who were two of the main design players on the M14 and my desk was adjacent to Julio as Al had retired and gone to work for Bill Ruger.
In Match re-condition with good handloads both are capable of MOA accuracy but in out of the box condition and milsurp ammo you are looking at 5 to 6" accuracy at 100 yards and this was what they were required to do at the arsenals.
That leaves the "as found" condition of the rifles and sadly excellent shooting Garands are quite rare and the "mechanics" to tune them to match accuracy are getting very hard to find as gunsmithing a Garand or a M1A is a skill rarely found these days in someone under 70 and most are 80+.
Gunsmiths that can rework bolt rifles are pretty much everywhere and you can even learn to do it youself if you take time to go to a local tech school and learn machine shop and get you own lathe and mill.
There are several excellent books on reworking bolt guns such as "Gunsmithing" by Roy Dunlap which I noticed when I was in bookstore the other day when I was in Williamsburg, Va is back in print for $44.00.
Another if The Complete Guide to Precision Rifle Barrel Fitting by Hinnant that are availabe from multiple sources on the internet and I think they are like $35.00.
First thing to do is find a gunsmith to keep you new toy going because if you do much shooting parts are going to wear and sooner or later you are going to get doubles or even tripples on either system. I have had them on Garands and M1As.
On the Garand timing is critical for operation. If your gunsmith doesn't have a timing block chances are real good he doesn't understand timing. Ask him how timing is restored and show you exactly on which part controls timing.
Ask him what is done to correct a enbloc clip and ammo jumping ship on the 6th or 7th round.
Ask him how he is going to adjust headspace on a new barrel. New barrels are headspaced short as barrels are not interchangeable.
You want a rifle that ejects hot brass between 1:00 and 2:00 and if everything is correct it will do so.
Ask him to explain port window envelope pressure on the Garand and suggest ways to determine it and why it is important.
Garands should not be fired with ammo having bullets weighing over 175 grains or with the slower burning propellants as some ammo will wreck your gas system on the Garand or the M1A pretty quickly.
Ask how the iron sights are "tightened" to prevent their movement when cranked high for longer ranges. The rule of thumb is the higher you crank your rear sight the more movement will be detected which will greatly reduce your effectiveness unless you have someone that can tighten it correctly.
Getting a Garand in 7.62 is a good move as that round does not tend to warp the op rod. All Garands have "bent" op rods. Bent too little it is not straight, bent too much it is not straight and it is a art form to get one correct.
When the military teams issued M1 Garands for match shooting in 30.06 they were issued with three op rods "gaged for straighness" as it took three op rods to get though a shooting season. With the conversion to 7.62 it was found the same op rod could be used the entire season due to a different pressure curve of the ammo.
Gas cylinders are getting harder to find and great care must be take to keep them from wearing and very few people know how to do this. The secret is to keep the carbon soft that is injected into the cylinder and the only way I have found to do this is put about 8 drops of Mobil 1 synthetic motor oil on the op rod, store the rifle muzzle down and let the Mobil 1 run down the op rod and collect in the cylinder. Mobil 1 will keep the carbon in suspension because the propellant leaves carbon upon firing and it is soft but when it cools it becomes quite hard and will cause extreme wear quickly.
On both the M1A and Garand cleaning them from the muzzle must be accomplished very carefully in order for the op rod not to be drug on the inside of the muzzle destroying accuracy.
This shows you what needs to be fabricated to prevent muzzle wear while cleaning:
http://superiorbarrels.com/Bore Guides/Bore Guides.htm
You can get one from them or you can make one yourself if you have access to a lathe and several sets of dies to for the necks. I have these on all my cleaning rods I use for gas guns.
First thing to invest in is a Muzzle Wear Gage. Fulton Armory and CMP have them and they are critical for determing muzzle wear because if your muzzle has been improperly cleaned and the crown is damaged scopes, handloads, bedding is not going to overcome this condition and will require a barrel recrown or a new barrel.
If you get into Tanker's you have a different ballgame as the gas port is about six inches closer to the chamber and the pressures are going to be out the roof. Fortunately you can buy gas cylinder plugs with vent holes to bleed off the excess pressure. I have one Tanker and it was a trick to getting it going right and I wound up making my own gas cylinder from stainless steel and it was "tuned" by opening the vent hole a few thou at a time and now I can shoot M2 Ball Ammo and LC/FA Match ammo interchangeably and the ejection pattern stays between 1:00 and 2:00.
I also had to change the op rod spring as the remanufacturer put in the wrong spring but fortunately Wolff Springs makes a Tanker Spring that works great.
If you are possibly considering a Garand or a M1A for survival applications you will need to add a considerable amount of "care and cleaning" items to you back pack to keep them going. Though I have both, understand both I would not choose either for a long term survival application or a accuracy application long term.
The point being that just because you can put a scope on it the chances of you getting a more accurate rifle is somewhat slim but can be done.