Actually, if the MXC tranny suits you, it's easier to add lights to the MXC (already has the lighting coil, right?) than mess with the tranny on the EXC. Sounds like the MXC is the best choice for you folks, even if you *do* want to ride at night!
This difference of opinion is really interesting to me. It could be that for agressive riders, the MXC tranny is better. I wouldn't consider myself to be very agressive, so I don't notice the gaps. In fact, when I'm out in the desert, I put the 14 tooth sprocket back on because I don't like having to shift so much with the 13 toother. (And the EXC 13-tooth is still wider than the MXC 14-tooth.) Plus, I could be a little lighter than other 380 riders (200 lbs no gear), and I'm on the 380 not a 300. It could also be the type of terrain we ride. I don't ride deep sand very much. Some of the trails I like to ride require significant amounts of time in FIRST gear, even with the 13 tooth front sprocket. One trail in particular requires a significant amount of time in first gear SLIPPING THE CLUTCH to keep the rpm up so you don't stall. (This trail is a favorite with trials clubs - very challenging in many ways.) I'ts nice to be able to ride back to camp on the dirt road in 5th with a a semi-acceptable top speed. The poor fellas with the MX trannies are tapped out at frustratingly slow speeds when they gear down for the trails.
Primary reduction refers to the internal gearing between the crankshaft and the gears. Final reduction (or final gearing) refers to the sprockets. I *believe* that the MXC and the EXC have identical primary reduction ratios.
Burt