I prefer probing, but that's because I feel safer doing it; I know I have a steady and delicate hand, and that I will be able to feel very well...whereas the idea of squeezing the snake until its insides pop out tends to squick me out. Besides the fact that I learned to probe first, and only saw a snake popped in person once, but wasn't being shown how hard to squeeze--I just don't feel comfortable squeezing such a delicate area so hard. Probing, to me, seems less stressful for the snake; mine always seem calm when I practice on them.
EDIT: However, as Sean said, visual confirmation is far better proof than a scale count which can be wrong. Still, the above is just my personal opinion; I might just be too afraid of hurting the animals, since I'm told that one CAN pick up a pregnant cat without hurting her, after refusing to do so for fear of it. XDD