they are both very good, ok ressurection is more of the same as the first with nothing much added, but its enjoyable and thats the main thing, I personally hope they make more, though a change of setting and story would have to be made to vary it.
I wouldnt say Ressurection is longer than the first, probably shorter if anything.
Both excellent gfx, great effects and surround sound, like whats already been said, play it in the dark and whack the volume up!
As for MP i've only played DM on Xbox MP and that is a lot of fun, so no reason why the PC's shouldnt be.