.NET perf can be great in the sense that it provides more tools to write C-like code while still taking advantage of safe memory management, as compared to Java. On the downside, both their JIT and their GC seem to be far less sophisticated than the JVM.

Why, for instance, does the CLR GC not have something like TLABs? The result is that it seems like .NET devs have to worry a lot more about the expense of small, short-lived allocations, while in Java these things are much cheaper.

Overall, I think it's easier to program idiomatically in Java and get decent performance out-of-the-box, while C# may provide more opportunities for optimization without having to rely on something equivalent to sun.misc.Unsafe and offheap allocations.