As someone who had both the Neo and Classics in my possession at the same time, with me having 2 pairs of the 99s (one I won, the other coincidental review pair), I can safely say that the clamping force will be almost identical between the 2 sets of cans. They both use the same ear pads, headband, and pad. There might be slight variations in each unique pair due to manufacturing, but there is no logical reason one would have less than the other.
As for the sound, the differences are subtle and have already been stated. Both carry the same sound signature with the same drivers and cup dimensions, the only difference in sound are the characteristics of wood vs plastic. The wood adds a unique distortion characteristic to the FR response, some may like it a lot (like myself) while some would prefer a more universally accepted sound.
All in all, it really comes down to your budget. If you can afford the Classics, I HIGHLY recommend going with them. If you can only afford the Neo, I guarantee you will be happy with the sound.