First off, there's a quote that comes to mind when I read some of the comments here: there are lies, damn lies, and statistics. While there may be more army casualties in Afghanistan than other branches, there are also more army *units* in Afganistan than other people on the sharp end.- as of 2008, there were 22,500 army solders, and 2,500 marines. Not saying the intent was to mislead, but let's get real here.
Secondly, what branch you should pursue also depends upon your goals. Are you looking to make the military your career, or just putting in a tour or two?
Keep in mind that while you can join either branch, there's no guarantee that you'll get the posting you're looking for, especially if you're going for an elite force like one of the airborne divisions. The recruiter can certainly give you advice on how to maximize your chances to get what you want, but (if I recall correctly) they lack the authority to make posting decisions.
I would urge you to do some research on the whole 'recruitment' process before you make your choice. I don't have any idea of where to start that, however.