I should caveat my answer with the following
(a) I am a former Microsoft employee
(b) I am married to a current Microsoft employee
(c) Microsoft has several of the largest LoadRunner virtual user licenses that I am aware of (anywhere) used for performance testing customer solutions in their performance labs (either Product labs or Premier customer labs)
I am a MSDN Universal Subscriber and have access to all of the tools Microsoft produces through direct license or through good friends who work for Microsoft.
Call this an endorsement if you wish, but I choose LoadRunner. It's easier to use. It has more direct support for a wider variety of protocols. The record/replay engine and API set is very rich. It's analysis engine is superior to what Microsoft offers. It's multi-platform (try doing an Oracle Load Test with Microsoft tools).
Some may point out that there may be only a couple of dozen more features that LoadRunner has that Microsoft does not, but that difference in feature set makes all the difference in efficiency in use of the tool and analysis of the results. Microsoft has a very direct tendency to "eat it's own dog food," that is "use it's own tools" wherever and whenever possible.
Microsoft has found a compelling reason to make use of LoadRunner for their testing efforts. So have I. Maybe for different reasons, but likely seeking the same efficiency in use.