There may be a slight difference recoil-wise with 308 vs 30-06, but IMHO the weight of the weapon is goign to have far more to do with the percieved recoil than the minor difference between the two cartridges.
Once upon a time I had this same concern. Then I realized it really didn't matter. I have a Garand in 30-06 and an M1A in 308, and they each do their thing well. I can't perceive much of any of a recoil difference, probably because the Garand is a bit heavier and soaks up the minor difference in recoil.
Don't waste your time trying to do cross platfom caliber acrobatics, just get the weapon you like. If you like the Garand and its en bloc clip, get it in 30-06 from the CMP. If you like the M1A with its magazine (it is basically a mag fed Garand) then get it and enjoy the price of .308.
To be honest, unless you really have a hard on for the M1A, just get an M1 Garand. You can get them for $400-$500 from the CMP, they are inspected for function (and they will fix it if you get one that doesn't work for some reason) pus it is a real USGI weapon that will go up in value as the years come and go. Ammo is reasonably priced from teh CMP, you can get 30-06 for under 25 cents a round if you buy 1,000 rounds of Lake City 30-06 from them. A great deal!