I think, in any of the situations or decisions that are to be made.. personally, I would choose which one I thought was best, and then enjoy the time as long as it was granted.. and when it got too consistently painful for the animal, I think that's when I would euthanize. I would want them to have as much life as they could, but I wouldn't want them to suffer. So..that way I've given them as much as I could of their life, with as little pain as possible. Of course, that would still leave the decision of whether to go with surgery, or not.
Sorry I'm not more help.