Speaking as someone with both diamond and platinum, I think the quality and value of the hotels you stay at are more important than the rewards program. If you wanted the rewards program that gives the most stays, then you would pick neither.
Also, the best choices to stay vary by location and price level. For example, I will almost always pick HGI over CY. I value MR points more than HH points, but not enough to stay in an inferior hotel.
Your cities and budget should probably decide which brand works better.