Here's our thought process...
First, if we thought we had the ideal candidates for Godparents, we'd probably use the same individuals for each of our children. As we don't consider anyone we know to be the "perfect" choice (as I'll explain shortly), we've so far gone with different Godparents for each child.
What would be the ideal? I think it would be a family member or incredibly close family friend to ensure that some degree of contact between the Godparent and the child continues to be possible as the years pass. Also, ideally they'd live nearby.
Now, the only Traditional Catholics in our family live 1000+ miles from us (and even those options are quite limited). For some Godparents, we've gone with family members eventhough they never see our children. For others, we've leaned the other way and chosen people we know from our chapel that may or may not still be in this area 5, 10, 15 years from now.
Given that, we do try and choose people who seem more stable and likely to stick around -- our chapel's coordinator, for instance. I wouldn't intentionally try and burden someone with trying to keep spiritual track of a child they aren't related to and never see.
Above all, they have to (at least outwardly) be a solid Catholic who we hope will take their job seriously and say a few prayers for our kids.