I took the YMMV to being the chances of having an open seat next to you in the rear cabin which is great advice for such a long flight.

I think most folks who have sat in the new JL J or UA J would definitely choose those seats if sleeping is important. Even the new AA J would be a more comfortable sleeping seat for a 15 hour flight than old DL BE, unless of course OP is an easy, anywhere sleeper. Then it wouldn't matter.

If there are other factors - mileage, corporate edict, etc., so OP has to/wants fly DL then BE is better than Y. Otherwise, it's worth looking at options IMHO.