Hi RoderickM
I think the simple answer to your question about which type to use will depend on your environment, but, as a general rule, you should use user CALs if you have more devices than users, and Device CALs if you have more users than devices. That way, you'll be operating legally, and having the lowest number of licences.
For example, if you've got 30 machines, and operate 3 X 8 hour shifts, you could have 90 users, so device CAL needed would be 30, user cals needed 90.
The opposite is if you've got 30 users, each of which have 3 different computers on their desk, in which case you need 30 user licences, and 90 device ones.
Exact requirements will depend on your environment setup, but as a general rule there seems little point in paying for licences you don't actually need, so go for whichever option will cost you less, either user CAL if you've more devices than users, or device CAL if you have more users than devices.
Hope this helps a little