Both firearms you mention have a host of aftermarket products designed at making them weigh a lot more. Some of them actually improve functionality or ergonomics, many are just for looks. The M14, however, enjoys a much more robust aftermarket than the SKS and there are a lot of genuine upgrades you can do to make the rifle more reliable, more accurate, and more precise. The SKS is not precise rifle, and it will never be one. It is a lot of fun, it's cheap, and you can "bubba" the crap out of it it out if you want. I did.
Check out the respective forums for these rifles. There is a lot of information here. Please read before asking, as the question may have been answered a thousand times before.