I used both. I had KMK signature + Optoprep. Looking back, I feel like KMK plus would have been enough. KMK is better at teaching you the material, but I think Optoprep questions are harder and prepared me better. KMK has a lot of questions also but my opinion is that they only ask/drill topics that they already emphasized a lot in their book and videos.
Optoprep without KMK would only be good if you have a really strong foundation already and good class notes.