All the RFS EXC/MXC four stroke KTMs are e-start, all are 6 speeds, all have sidestands.
MXC has a bigger tank that actually incorporates the shrouds, a close ratio tranny, no headlight, and (pretty sure about this) different suspension valving but same spring rates.
Since, apart from the transmission, you could easily convert one into the other IMO it deserves special consideration.
Here are the tranny ratios for each:
MXC
16/32
18/30
20/28
22/26
24/24
21/18
EXC
14/34
17/31
19/28
22/26
24/23
26/21
Note that the EXC is only slightly taller in 5th, but much lower in 1st (6% taller up top vs 18% lower in first), other ratios are close, 4th gear is actually the same. I'm fairly certain final gearing is the same (14/50??).
First four ratios on the SX are the same as the MXC (the SX actually has the MXC 5th & 6th gear wheels already installed on one tranny shaft from the factory).
Verdict: For tight woods I'd definitely want the slimmer tank and low first gear of the EXC, out west the close ratio and bigger tank is the ticket (but the EXC still seems to be more popular even out here).
Hope this helps.