I'd say if you're after a solid FPS set in the modern era with unparalleled use of audio/video for the DS then yah it's a game you'd want. The controls are the standard more/less since Metroid and Brothers in Arms.
I'd say both it and BiA are challenging more or less, but I think COD4 has a few more moments that are just nasty to get through making it a bit harder.
I'm a WW2 shooter lover for sure between COD1-3 and the console end of Medal of Honor...so it kind of gives a nod to BiA in that respect. Also I think the more open stages and detailing of them in BiA is better, BUT...BiA has this stupid marker system where you literally have to run to a checkpoint on the map in a more straighter of a line than CoD4. I maybe over examining it though as COD4 has it too on the map and it's just presentation 'how' so I'd have to doublecheck and I'm not in the mood right now to do so.
If you though are into doing more than just a hop on a tank/jeep + on foot type thing then COD4 wins. COD4 has multiple stages where you get to strafe a lot of enemies within a Blackhawk. Also there is one stage in a Hercules at high altitude using a high calibur bullet gun, but also a 100mm cannon that can drop buildings and it's fun to drop them and vaporize enemy heat signatures on the ground within.