Why? Ten shields are ten shields. If units are what you need, and units are what you are building, and you have forests that should come down, and you can spare the worker turns, chop away.
It is easier to chop toward infrastructure because in general it costs more so it is easier to time the chop to make sure you aren't wasting it, but other than that, I don't see why you should prefer chopping infrastructure to chopping units.