I'm not sure. I only played a little of both. Pacific Assault and MW2.
I liked the smaller features that made PA special. Like the training missions and healing people etc.
What I liked about Modern Warfare 2 was the pace of it (yes the snowmobile).
It depends on your taste. PA had a steady pace, and it was a good shuffle of the tasks you had to carry out (e.g. shoot down plane, get person to ER) It had a good mix.
MW2 was up and down. But had a rather cliche story. One minute you're a secret agent strolling through a mall killing dozens of civilians (which is enough of a risk as you're working for the government) but then somehow they find you out and all that manslaughter was for fuckall.
I'd say MoH over all now that I think about it. It's less hyped and less overrated meaning less mediocrity hopefully. If all else fails give halo reach (is it?) a shot!