I currently use VSS, and have also used MKS as well.... SourceGear has got a new one out called SourceVault that takes the best of both, and puts them into a "familiar" format (ala VSS) and uses a SQL Server proper for it's storage (As opposed to VSS's proprietary storage.) The advantage I see in that is backupability. I haven't had the opportune to try it just yet, but I plan on it.

Tg