Could you get one at launch, then get the other a few months later? If so then i'd go for Ghosts first. The first few weeks of CoD are the best, before people start learning exploits and ranking up, whereas in battlefield everyone has a chance of winning regardless of your level.
Normally i'd just go for CoD and forget Battlefield, but this time it looks pretty good. Plus Battlefield campaigns are usually pretty fun, so getting both seems like the smart idea.
This is how im feeling aswell. Mw2 was probably the most fun i've ever had gaming. After barely playing Mw3 and skipping Bo2 all together, i hope ghosts can bring the same sense of enjoyment that Mw2 did.