Well I've had a solid late-night binge-fest with the new 'Battlefield; Bad Company 2' on the 360... just long enough to figure out that I like it a LOT better than 'Call of Duty: Modern Warefare 2'.
Don't get me wrong - COD:MW2 is what it is; a highly popular, buggy, twitchy, frantic frag-fest -- and it can be a lot of fun.
But, the more I play MW2, the more pissed-off I become with it. There's a lot of 'little things' in the game mechanics that all add up to a frustrating, limited experience.
Enter BF:BC2. Clearly they knew MW2 was going to be the benchmark against which their game was compared, so they set about strengthening their game in many areas where COD falls short.
For example; in MW2, say you just emptied the bulk of your clip into someone's face, and there's more enemies closing in on your location, fast. We've all been there. You know you can't sit still -- they'll be around that corner any second now -- but you first need to reload in preparation for the onslaught.
You'd like to initiate two actions at once -- a simple reload WHILE sprinting to better cover. However, in COD, the sprinting action cancels out a reload action, so you can't do both at the same time. As you fiddle with this, tangos close in and spray your location; their bullets zip through the nearby cover as if all the walls are made of paper. Rinse, Re-spawn & Repeat.
But in BF:BC2, You can run (and you can sprint for more than 3~ seconds!), AND reload at the same time. In addition, almost all structures in the map are 'destructible'; You can blow apart buildings and mow down trees. IMO, this is the 'missing piece' in most 'war games', and BC2 is worth playing just for this addition alone. Throw in the vehicles; tanks, atvs, helicopters (¶chutes), boats, etc.... and the gameplay experience is much more well-rounded; it transcends out of the COD realm of 'stylized arcade shooter' and into something much more akin to a 'real-world soldier simulator'.
Highly recommended if you're into the whole FPS-frag-fest fad.
TL;DR: BF:BC2 > MW2