SP only for me, at least until the DSL service here expands a bit and is able to reliably meet the capacity demands.
Also, for me I don't game every day, and I will go a week or two without even looking at my Xbox, so even if I am able to get online, I'm not sure it would be worth it for me.
I always play the Campaign mode and go through all of the levels in "order," in Single Player first, before doing online Multiplayer.
I like following the story-line step by step, before jumping in to online Multiplayer, where they could throw you into the last couple maps and "spoil the ending" if you will.
That, and Single player is a sort of "Baptism by Fire" (literally) when you go it alone, and so you are better prepared for online games.
I normally either got straight to MP or play the first mission and then go to MP. With SS, I went straight to MP. I do enjoy playing the missions in games online with friends in coop, though.
i make sure i play through the entire single player campaign before going online. it's my belief that a guy online that doesn't know the layout of a mission is not very helpful.
I never, ever play SP. Haven't played a singleplayer game since I owned a Sega Mega Drive :-) I only buy games for the MP, and this one is no exception.
I always play the campaign first before multiplayer. I do however play alot of online co-op with my friends after i beat the SS campaign becuase I do not really like the versus multiplayer options, sometimes they become too laggy and jittery, but online co-op is just fine for me.