I don't have a good feel for the numbers yet, but a +1 here or there does not seem a big deal when unit defences vary between 0 and 22+. I guess armour varies a lot less, so maybe upgrades are more of a help against missiles?

I tend to view one off payments for troops as almost irrelevant - the key thing for me is the upkeep. Hopefully your units stick around for a long time, so the cumulative upkeep will dwarf the purchase cost. Upgrading a unit rather than building a new one may make sense if you want to keep your total upkeep under control.

But generally, I would only upgrade where convenient, but unless it was a depleted veteran unit I would generally not prioritise it over investing in buildings or new units.