I have a few categories of interest: heat up time, hit size, conservation, portability, reliability and ease of use.
Hi there. Let me preface by saying I own an UD, and a close friend has a DBV (per my suggestion, because he wanted something relatively hands free to hit while gaming).
Heat up time -
UD: Nonexistent because its always on.
DBV: ~5-10 minutes. However, I remember reading somewhere that 7th Floor had a heating element turned on for years in their factory that hadn't failed yet. I don't know if that's recommended, though.
Conservation
UD: wins this one easily. The UD can hold quite a varying amount, depending on how deep you make your bowl, and how full you load it. Log vapes are known to be very conservative, and accurate with their dosing. If you're running low, you can load as little as you want in the bowl; the UD isn't picky.
DBV: ...isn't a very good at conserving. Don't confuse this with being a bad vape, because that's definitely not the case. It just requires you to load up quite a bit if you want to have a satisfying experience. I forget how much by weight an average DBV bowl is, can someone chime in? Just know it works great, but you're roped into loading up a fair amount per bowl.
Reliable
I'd say both vapes are proven to be reliable thus far. Both well made, and have great customer service if something does go wrong.
Ease of use
This is a difficult question.
DBV: great for the fact you can load a fairly large amount, and hit it many times without reloading. So it shines in group sessions, or while watching watching a movie, etc.
UD: Overall, just very user friendly. Design and use is really simple. It's always ready to go, and I leave mine out on my desk because I also diffuse essential oils with it. I would give the UD the edge here too, just because I can get a session going in about 15 seconds, flat.
Both are great choices, but I think they are aimed at different parts of the market. Do you usually partake by yourself, or with a larger group?