Tough to say, as there are a lot of factors to consider. Kant is certainly mainstream enough that I don't think you need to be too worried about it. If you think your Kant piece is going to be the better piece, then by all means stick with it. It's not going to help you to submit a worse piece of writing even if it aims to resolve some current problem.
For what its worth, my writing sample was a technical essay on Kant's theoretical philosophy, and was admitted to a couple top 25 programs (and, as it turns out, UCSD wasn't one of them).