I'm not sure if PDZ is the same, but each time you increase difficulty level more secondary objectives are added (on top of more and better AI enemies). I played sp first anyway, but on the default setting. When playing it co-op, up the difficulty setting and it gets a little tougher and more interesting, as well as offering some new things to do. PDZ is co-op done absolutely right over LIVE. It also runs exactly like the off-line game, which is very smooth.