In regards to one-part vs. two-part putty... here are the facts.
Fact: One part putties harden, or "dry," the same way that paint does: the liquid part (the solvent) evaporates away, leaving behind the hardened solid part. Since all one-part putties harden this way (via evaporation of the solvent, leaving behing the "putty" part), they by definition shrink as they dry– because they lose a part of their volume (the solvent) as they dry. This is an irrefutable fact.
Fact: Two-part putties (the kind where you have to mix the resin and the hardener) don't dry via evaporation. In fact, they don't "dry" at all... they set via a chemical reaction between the resin and hardener, the same way epoxy sets. Once this reaction has finished, the putty is as hard as it'll ever get, and it won't shrink two months down the road. There is no loss of volume because the putty has hardened via chemical reaction, not evaporation.
However... that's not to say you can't get perfect results using one-part putty. You can, if the putty has completely dried before you prime and paint. If you laid it on thick, after a while it may seem dry, and even sand well, but deeper down the solvents have not all evaporated away, and in a case like this you will see evidence of shrinkage marring your finished model down the road.
That's the pitfall of one-part putties. They do shrink (and crack if applied too thick). If you know what you're doing and have had experience using one-part putties, and you know enough to let the putty dry completely before finishing, yes, you can get good results. Many modelers, especially beginners, don't know that, and they're the ones most likely to have the problem of visible shrinking down the line, after the model is finished. And of course, there's nothing you can do to fix it at that point.
Since two-part putties don't have this problem to deal with, IMO they're the better choice. A two-part putty, if mixed in the correct ratio (approx, 10 parts putty to 1 part hardener) will not shrink or crack, ever.