Depends on how you promote your storefront.  If you just leave it up to people finding your items through a broad search query well good luck on that.  You can start out with the free one and give it a try to see if you should bump up to a paid store if you can generate sales .    Just grab the templates they have and match your works to them and give a few items a shot.  Think most are 200dpi then the various sizes per item.  Doesn't hurt to give it a shot anyways with the free one.