If you want a decent answer to this question, I'd go somewhere else and ask it. This forum is (unfortunately) known for its vicious patriotism toward RO and against all other games, whether they are good or not, and judged solely on the basis of "how realistic it is compared to RO".
Not a bad thing, as RO is great, but if you want a seriously unbiased opinion, ask it somewhere else.
If you want my two cents, both games you mentioned are really great products in their own right. DoD:S, while a bit arcadey and goofy sometimes, is an absolute blast. CoD2 is alright, but as others stated, most of the CoD players stuck with United Offensive, so there are more players playing that.
What would decide it for me is customizability. DoD:S has tons of custom models and skins to make things, in the visual department at least, just the way you want them. While you can do this with CoD/CoD2, you can't play on hardly any servers with custom content, and the ones you would play on would be rampant with aimbots and wallhacks.
I'd go for DoD:S for multiplayer, hands down.