6) Relatively more hazzle when updating, and when reselling the system, since you would have to change registration data (I assume that this *is* possible?).
This is actually pretty easy for OS4. You can "unregister" your copy through Hyperion's site and the new owner can then re-register it with the same code as if it were a new copy.
With a second-hand MorphOS machine, you'll have full functionality and upgradability out of the box, but you'll need some manual intervention from the Dev Team to put the keyfile in your name.
Six of one, half-dozen of the other. Not sure what's better in this respect.
