I prefer the feel of the Classic myself. I like the softer rounded edges. It sort of feels like a half used bar soap in the hand - very easy to hold on to and it has nice gentle lines. The slight flare at each end fits my hand very well.
The plain model is a fine knife as well but, I find the angular cut away for the release a little bit of a drawback. If my hands were bigger or smaller, the feel of the knife would be different and, perhaps it would be my preferred knife.
Which one is preferred? I think it's mostly a matter of personal preference biased slightly by the phyisical attributes of your hand.
Stay Sharp,
Sid