The higher the Buffer and the higher the RPM, THE BETTER

The Buffer size is there to help the HDD READ information faster.
Therefore, the larger the faster it will read. Don't get confused with WRITE and READ. Again, the buffer is just for read speed. Think of a buffer as RAM stored within the HDD.

RPM (Revolutions Per Minute) refers to the amount of times the spindle will make a complete 360 in 1 minute. This also is for read speed. So if an HDD has an RPM of 7,200 that means the spindle will rotate 7,200 per minute.

I would recommend the lower RPM speed (7200) and a higher buffer size (32mb) as the higher RPM (10,000) will tend to heat up very quickly and might actually burn your HDD. And you don't want that do you haha.
What you should really look at in a HDD is its Average Write Time. (measured in MS). The lower the MS the better. For example, a HDD with a write time of 10ms is better than a 12ms. Also make sure that they're all using a SATA interface with a 3.0Gb per second transfer rate.

Hope i was of some help, BEHATSLAHA AHI