Kevin,
it seems as though some things have changed:
The 98 and 99 MXC and EXC had different fork rates, however, the new ones are now the same. The EXC has always had a 6 speed, so why they suddenly post 5 speed is a mystery (perhaps it's true, perhaps a misprint).
I know a number of people who ride EXCs and a couple on MXCs. The Canadian and US spec is the same. This is what I and my friends have actually experienced:
The EXC and MXC with good gearing is 14:48 and the EXCs 6th is higher geared - I find the EXC is a little slow occassionally but livable and the MXC is just too slow to stay with many of my friends when we get onto fast sections. The power is good and the bike revs out fully, but it just aint fast enough. This is not only my opinion but that of numerous riders/owners.
This still must be kept in context though, how fast do you want to go ?
As for lighting, the EXC has more Wattage output and yes the MXC coil will run the EXC lights. However, even the EXC has lousy lighting. Many people are looking for more Wattage. The standard EXC lighting will just about get you home at dusk if you are carefull (the MXC is anightmare). Anyone doing night time riding goes for aftermarket 200 Watt kits. In Europe people put the GS lighting coil on, which is just a little better than the EXC.
So it seems as though the only real questions would be:
Have they really changed the gearbox on the EXC ?
What kind of lighting do you need - But regardless you could easily modify that.
The MXC still has a slightly lighter flywheel and slightly more agressive ignition curve - Great for MX, harder work on the trails and in woods.
Deffinitely both great bikes !!