I've had plenty of Delta Dollars over the years (including one glorious quad-bump 2-day adventure), and just as with NW DB-210s, they each have their benefits. For a long time, DDs could be combined and gifted and only used in person, while NW VBDs were owner-use-only and non-combinable and online (very much prefer the online part!). I suspect when DD go fully online, they'll be only usable by the purchaser (with maybe the benefit of the entire PNR). No idea about the online combinability. Even if online DDs are available to gift, I suspect that'll go away over time.

As per 006 and 012 questions, the NW thread is just about all posts from OP and me. Good reading, actually. The joy and fun of issuing-carrier vs. segment operating-carrier.