i think someone on here did a comparison and it actually wasnt quite the same "feeling" as the snap on ones. i personally have the williams ones that your talking about and i love them. never had any problems with them and to me they feel the same as the snap on ones.
There are differences but, they are very minor aside from the differences in their Ebay prices but, remember you're buying a Williams screwdriver set not a Snap on screwdriver set.
I have both if you took off the snappy name and you took off the williams name, then I could never tell them apart. Both are wonderful screwdrivers you will never go wrong by either brand.