KMK I would consider the backbone of your studying. Optoprep was great for hitting some key points and there were definitely some things that I learned through Optoprep (that I saw on boards) that I didn't learn from KMK. So if you use both, you're golden. Optoprep was great for quick questions after I had been through the KMK stuff.
I would highly recommend both, but KMK is the best no doubt.