In a nutshell, the "plus" pickup system is easier to use and sounds better. Please see this video that I made at Martin to demonstrate its ease of use:
Also, on that guitar's page on our website-
- you can click the "guitar specs" button to see all the specs. Basically the loss of ebony for the fingerboard and bridge is the main change but I can't say why they changed it, you'll have to direct that to Martin. They can be reached at 800-345-3103. Best of luck. Thanks!