With F-15C you are stuck with A2A, while with F/A-18C you can do air to air and air to ground, so more opportunities to have fun. Yes, you have campaigns for F-15C, there was a new campaign released recently. It all depends on what you like, really. On the other hand, the F-15C will cost you 10 dollars, on some sales it is 5-6 dollars, so you won't pay too much anyway.