Dickerson Mine > Lewis Morris.
As with all trail systems, the most important factor for MTB access and overall trail conditions is the relationship with the land manager. Mine Hill Township is the land manager for DM, and from all appearances has a great relationship with JORBA and the trail builders. This shows in the quality of the trails (both for MTB and overall sustainability). Contrast with LM, where relationship with MCPC is not good, and quality of trails is not good and is overall declining (for both MTB and sustainability). Contrast again with Watchung Reservation, where relationship with Union County is adversarial (at best), and there is no legal MTB and trail conditions are overall abysmal.
This is why MTB (unlike every other trail user) has to put its best foot forward whenever possible for trail access and sustainability.