All the fillers tend to stiffen over time, so buying a lot isn't a good idea unless you are going to get through a lot of it, which modelmakers don't tend to do.
I use Upol Dolphin glaze and Fantastic, Dolphin glaze is more like a two part stopper, so quite runny, I sometimes mix the two to form an 'inbetween' filler. You can purchase each in bag form rather than in a tin, this helps keep the product fresher.