Welcome to JOF!
I tend to agree, the 800lb rated WDH would be desirable because I think it will be easy to slip over a 600lb loaded HTT tongue weight under certain loading conditions, and IMO the 800lb rated WDH wouldn't be an overkill under proper adjustment.
Check the yellow sticker on the side of the 2015 X18D for the actual "As Shipped" UVW, it will be heavier then the published 3,410lbs......., probably adding another 300lbs. At the end of the day the X18D loaded for family camping could easily come in at 4,400lbs (no fluids), so a 13% to 15% tongue weight range would be 572lbs to 660lbs.
If you decide to go with the 600lb rated WDH, a visit to the CAT scale may be desirable to confirm your loaded tongue weight doesn't exceed 600lbs, especially if you tow with any fluid in the HTT tanks.
CAT scale how-to: http://www.jaycoowners.com/forums/f3...v-tt-3871.html