Eating counter service vs spliting a sit down meal, as far as cost is concerned, really depends on what kind of eaters you are and what types of things you tend to order. If you've got teens with you (especially BOYS) they'll eat whatever you put infront of them, lick the plate clean, and be hungry again an hour later! But if your group tends to eat light, then spliting sit-down is a good way to save money, I'd think you'd break even with counter service in most cases. In my own experiance, girls and women can split counter service very well, but boys and men tend to eat the whole thing with no trouble. But I'm sure that's not true of everyone!
BTW, no mater what, try to plan on at least one or two sit down meals. Burgers, hot dogs, and chicken strips get old after eating them all week!