I would say that the single payer is similar to COD4 story wise(obviously as it follows on) but with better weapons and much better graphics (maxed out).
Some nice things in the SP like on the snowy mission, but won't spoil it. I am thoroughly enjoying it.
I would say that it beats COD4 simply due to the fact that it is the same as it, if not better in gameplay, and tons better in graphics (Dare i say, some of the Favela scenes remind me of Far Cry 2 style graphics?).
On to the MP. I haven't had any problems at all so far. As soon as i press Find Servers, it does it and i am in. I am doing quite well online, better than i thought as i'm used to the 360, so that is good. Also the graphics for the MP are very nice too. Good explosion physics etc. The blood effect is a nice addition too. Some kill streak perks are annoying, such as the guided missile, but yea, all in all, a good MP experience.
Now, which is better, SP or MP. I'd have to say that both have their strengths, and very big ones at that. I am thoroughly enjoying both at the moment.
The only problem that i have seen so far is that, i am already near the end of ACT II (out of 3) of the SP campaign after around 2 1/2hrs play time. Oh and also, the mic capability doesn't seem to be working in MP yet?
Any more questions?