Assuming you're interested in preventive maintenance, for anyone with a regular route/routine or a cyclocomputer, or even an old-fashioned mechanical odometer, mileage is probably the 2nd easiest monitored value. The easiest would be to go by the calendar, swap every nth month. A bit arbitrary, but very clean cut.
All types of measuring does open up for some sort of debate, and of course, you have to get the measuring done, be confident of your readings ASO.
Certainly "better", in terms of really getting max life out of each chain. But for the fairly inexpensive KMC chains I usually run, not really enough to matter.