If you've got the money, I'd also go for the M1. Yes, .30-06 ammo is more expensive than 7.62x39, but the M1 should be far more accurate and will certainly appreciate in value faster than the SKS.
If you reload, then the ammo cost drops. If you cast your own bullets, the cost plummets. As far as ammo damaging the rifle, that's true, a stock M1 has an envelope for gas pressure that modern off the shelf ammo probably exceeds, but you can get an adjustable gas port that is a non-permanent modification to the rifle that will enable you to use pretty much any .30-06 round out there.
I cast my own bullets from wheel weights and load to around 1900fps with gas checks. The action cycles just fine and it's accurate out to about 750 yards. Beyond that, I use commercial plated bullets. Also, CMP sells M2 ball ammo relatively cheaply (relative, of course, depends on your budget.)
I've got a couple of SKS's, too. They're fun, short to medium range plinkers. They're not terribly accurate - within their range, they'll hit what I aim at, but they are military rifles that were made for a different sort of combat than the M1. And, unfortunately, ammo for them is a little spendy right now, too. Once my (low) stock of milsurp ammo runs out, I'll probably start casting and loading those, too.
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