This is an interesting question. I haven't yet done any performance testing between these two runtime environments using NCrunch, so at the moment I'm not well informed enough to say which one will execute tests faster.

In terms of reliability, I would err towards .NET Framework for the simple reason that this framework and the NCrunch implementation for it have both been around for many years and are both extremely well tested.

In terms of effectiveness, I would say this depends on the intended purpose of the software you're developing. If this is for libraries you intend to publish with the intention of being cross platform, it may be best to multi-target your test project to ensure you have thorough testing on both platforms. If you do eventually intend on having a primary platform to run your code, definitely use this for your test project.