I seriously doubt that an SKS will ever be a collectible as a Garand. For one thing, there were many more SKS' made. Heck, rumor has it they are still being made.
Garands, OTOH, are pretty much finished when it comes to manufacture in mass quantities. The CMP is getting close to the end and the price will only go up, unless the Koreans (which are pretty ragged from what I understand) are allowed back in the USA.
Overall, I would think that the Garand is a much better investment. And I would highly recommend you get one from the CMP rather than buying on the secondary market.