As Gertsy said, 6.2 in a newer and improved motherboard. However, if you're planning on adding a 3.1 ROM (instead of living with KS1.3 or 2.0) then I'd look at the condition of each machine, number of floppy drives, any expansion cards included (SCSI?) and the condition of the battery.
6.3 is better if you can find one as it is ECS instead of OCS.
