3.5" 15k RPM drives vs 2.5" 10k RPM drives

You should be concerned with head seek time and transfer rate. It is true that they depend on the form factor, but they also depend on many other variables. Looking only at the physical size and ignoring those variables would be wrong.

With this in mind, let's compare the most recent versions of some widely used disks: Seagate Cheetah 15K.7 and Savvio 10K.3.

• Random reads and writes: avg seek time of 3.4msec (read) and 3.9msec (write) vs. 4.2msec and 4.6msec, latency is 2.0 msec vs. 3.0msec. Therefore Cheetah should hit 185IOPS read and 170IOPS write, Savvio will do 140IOPS read and 130IOPS write.
• Sequential reads and writes: looking at sustained transfer rates, we see 122-204MBps for Cheetah and 67-124MBps for Savvio. These are maximums, the range is so wide because of different amount of data per track on the inside and the outside of the disk.

Bottom line: 2.5" disks are comparable to 3.5" disks for random operations, and usually you can compensate or even win with 2.5" due to the larger number of spindles in the same physical volume (disk array or server). However, if you need to pump a lot of sequential data, 3.5" disk is still the king.

Prior to me starting my current job our ERP package had been moved to a new server by a predecessor, who had intended to decommission and scrap the old machine. The performance was appalling and both the vendor and that predecessor thought it was due to a configuration error, although they never found the cause. After taking over the job and examining all the factors it was clear to me that the 2.5 inch 10K SAS drives on the new server simply couldn't keep up. I therefore moved the ERP package back to the old server, which has 3.5 inch 15k SCSI drives. Performance is now back to where it should be. In both cases the drives are arranged as a 3 drive RAID 5 array.

On the subject of head movement actuator others have mentioned, they have failed to take into account that the actuator in 3.5 inch drives is generally considerably more powerful than that in a 2.5 inch drive. The consequence is that although it needs to travel a little further (about one tenth on an inch or less) it can do so more rapidly. This negates the perceived advantage of the smaller drive.