5400 RPM or 7200 RPM NAS hard drive for my new DiskStation DS918+?

I installed the 2nd 4GB stick, i.e. I now have 2x4GB RAM installed in my DS718+; with that, I had hoped it would resolve playback issues with certain recordings but, unfortunately, this hasn’t come true - see new thread issues-playing-certain-recordings-on-ds718-and-apple-tv-4th-gen