It is a fact that collies are an older breed than shelties, but the original collie was more a border collie type dog than today's collie.


The Shetland Sheepdog, like the Collie, traces to the Border Collie of Scotland, which, transported to the Shetland Islands and crossed with small, intelligent, longhaired breeds, was reduced to miniature proportions. Subsequently crosses were made from time to time with Collies. This breed now bears the same relationship in size and general appearance to the Rough Collie as the Shetland Pony does to some of the larger breeds of horses.
Today's sheltie has collie ancestry so I don't see how they can be the older breed. It was collies from Scotland that were imported to the Shetland Islands where the sheltie was "developed." The scottish border collie was the original collie and later the three evolved but the rough collie in any dog breed book where it lists the era of origination always has an older one than the sheltie.

Taken from here (as well as read in many many other sources as I was highly involved with collies growing up): http://www.akc.org/breeds/shetland_sheepdog/index.cfm