Obviously, since I don't have my WBBB yet, I have not laid in it, but the general consensus on this forum from what I've read is that the WBBB provides a flatter lay. With that being said, I was able to lay somewhat diagonal in the NX-250 and could tell a noticeable difference. I am 6' and had tons of room on the foot and head end both. I only had about 10 minutes, so I didn't have a lot of time to try various positions hanging or laying.
From what I've read, the NX-250 does have quite a bit more room than the other models Clark offers. Some of that may be perceived room from the spreader poles.