As Jesteinf said, it is hard to help without knowing what you are looking for.
If you've already narrowed the choice to one of those two restaurants, you probably have something specific in mind with regard to food, atmosphere, etc. It would help if you told us something about how you got to that point.
Also, if you haven't already, check out the on-line menus. If there are particular dishes that you know will appeal to your parents, that's a good way to decide.
For what it's worth, I like the food at Blackbird more. But I hate being so cramped in and so I always ask for a table near the wall on the west side of the building where there is a little more space. My parents cannot stand the noise level at Blackbird and love MK.