I wish to use it to identify the computer to generate a software licence.
FYI, neither MAC addresses not serial numbers are perfect for software licences:
In many cases the MAC address is user configurable.
Macs can ‘lose’ their serial numbers when the logic board is replaced [1].
Logic board replacements will also result in the MAC address changing.
You’ll find that any other approach you take has similar problems. Much of this territory is covered by Technote 1103 Uniquely Identifying a Macintosh Computer.
My only concrete recommendation on this front is that, no matter what approach you choose, you implement a strategy for the user to recover when things go wrong, because things will inevitably go wrong.
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[1] Well, the Mac still has a serial number, you just can’t read it programmatically.