Adding a c15 would be to take advantage of the Li batteries, and light, while maintaining the advantages the C12-15 have (brushless electronic clutch, etc)
Adding a CSX for the obvious. Lightweight for all day type use. I've got some install jobs coming up at work (I'm not normally an installer, but am helping that team). We're assembling furniture with loads of screws and small, light, and ability to not overdrive delicate screws is what I'm after.