I've played CoD for the PC (version 1) and CoD 2 for both my PC and my Xbox 360.
I thought the original CoD was innovative in the use of the three storylines. That you start off the Soviet/Russian storyline with no weapon... some people thought was a gimmick but I thought it was innovative. Each storyline had a different feel to it.... the wide-open battles in Africa versus the scramble from building to building in the Russian campaign. CoD 2 for both PC and XBox 360 introduced a decent use of smoke, sand, etc. to limit visibility and provide concealment at interesting times.
However, I was just telling some friends yesterday that after battling thru those Rainbow 6 missions it has been much easier going (almost too easy) playing CoD2 for my XBox 360 again. In R6 I pretty regularly had to repeat a mission because I got killed....in CoD2 I did a run of 4-5 missions with barely any difficulty at all.
I miss being able to setup single envelopments etc. with teammates as one can do in R6 but it was fun to drive a tank across the desert and try to shoot Panzers in the hindquarters. Sadly, the graphics for this effect are not as good on my XBox 360 (kinda bland actually) as on my PC version of the same game.
I played R6 and CoD2 for XBox 360 at the "easy" settings. It is possible that CoD2 is more fun, more on par with R6 in some ways, at a harder setting.
I also paid $20 for CoD2 for XBox.... bought used at GameStop. It is amusing enough for $20 but I'm glad I didn't pay $45-55 for it. My wife bought me Rainbow 6 (she's a great gal, isn't she?) so that was free to me but even so.... I wouldn't have minded shelling out some retail $$$ for that one.