Hi Stephanie,
What you are asking is which source control clients are supported by TestComplete. Bascally anything to can understand the Microsoft Source Code Control API (MSSCCI).
Please keep in mind that the following has nothing to do with one source control solution over another but with the TC's source control integration.
We were using a supported client (Tam Tam CVS SCC) for a while. The problem was that the source control integration had an unberable performance hit on TC: in most extreme cases typing would only work at a keystoke speed of one every 3-4 seconds. This occured on VMware virtual machines that were quite distant for the source control system. As we didn't have time to investigate the issue, we just removed source control from our TC solution. Now we use a CVS client seperately from TC and everything works just fine !
So if your team requires TC source control integration and that you have options, I would suggest testing it in your context.
Sincerely