Easy selection method
An easy way at the moment to decide for an SMB requirement where a single SAN is going to provide all the shared storage is to ask "will you need bulk storage/archive/backup to disk?".
If the answer is yes, the capacity uplift you get from 3.5" SATA/NL-SAS disks capacities being up to 2TB compared to ~500GB on 2.5" makes the decision relatively easy.
Double the bays with 2.5" chasiss x 1/4 of the storage density = half the capacity of a disk 3.5" solution. Spindle count tends not to come into it for bulk storage, but you do use an extra bay to go RAID6 for high capacity disks, but you still come out ahead 9 times out of 10.
So, if you need a mix of fast and bulk storage, the SAS disks should be 3.5" - if you don't need bulk you can go 2.5"....