It is pretty useless if you want to win battles, but for XP the C is the best one. I spaded all A-10’s and the C got me the most XP, but I do agree that it needs to be put in a closer spawn especially on the bigger maps.
If you drop the GBU’s at about 25 km distance at 400 meters they’ll all glide in and hit, you just have to stay alive long enough. I got a lot of XP by farming this way.