You should use System.Diagnostics.Stopwatch, and you might want to consider a large sample. For example, repeat this test something like 10,000 times and average the results. If you think about it scientifically, it makes sense. The larger the sample, the better. You can weed out a lot of edge cases this way and really see what the core performance is like.
Another thing to consider is that JIT compilation and object creation can definitely skew your results, so make sure that you start and stop your Stopwatch at the appropriate times, and call the methods you want to test at least once before you begin your tests. Try to segregate just the parts you want to test from the rest of your code as much as possible.