I'd say that enough gravity to get most stuff to stick around but weak enough to allow easy escape would likely be ideal for most purposes, so a range of around Earth to Mars (maybe slightly lower) would be ideal

It would seem to depend on which resource was most desired.

My speculation is similar to yours. I can think of all sorts of reasons for and against mining earth before asteroids but for our purposes we don't really need to know. "All else being equal" instead of "no reason for any civilisation ever" conveys the desired message without confounding technicalities.