TFS scales very well particularly in the environment you described - mixture of onshore and offshore development staff. In particular the proxy server ensures good performance for the remote teams.
Furthermore, there is very good security features that allows granular access - and a good "alert'ing" system - say you want to limit what branches your offshore partner has access to and get emails every time they check into a particular branch you are only reluctantly giving them access to.
The environment is all integrated both in the VS IDE and an excellent web access portal (ie the former Team Plain - not Sharepoint). All of this is incorporated in a single tool (as opposed the need to integrate multiple different Open Source tools). It is well documented and supported with a large community around it.
But the biggest benefit is that all of the data is incorporated into a Data Warehouse. You should be able to track the productivity and more importantly quality of your various teams by seeing how much work they are getting done (task completion) check-ins, bugs, broken builds, failed tests and so on. How would you capture that with the various tools?
Furthermore, if you do want to stick with particular open source (or otherwise) tools, such as nUnit over MSTest, nCover for code analysis over Microsofts, you CAN do this with extensibility - even publishing the results in the warehouse.