I can give it a try. On vapor density the UD gives the thickest hits I have experienced with any vaporizer. The Solo gives the thickest hits I have experienced for a portable but it is hard to compare it to a home vape like the UD. Bowl size is also hard to compare as the UD has an adjustable bowl size.
For efficiency, the UD will once again top the Solo. I usually do one full cycle with the Solo, whereas 2-3 hits with my UD will leave me nice and medicated(I feel this is due to the thickness of hits from the UD). The Solo will definitely get you where you want to be it will just take a little longer to get there.
For taste, these two units are about equal IMO. Both offer great taste and a very pleasant vaping experience.
Both units are easy to use but in different situations. The UD is a great home vape. Load the stem, connect, and draw. Couldn't be any easier. The Solo is also the same routine but takes a minute or two to preheat. The solo is perfect for on the move, going over to a friend's house, or chilling on the back deck. Not quite stealth enough to be considered a stealth vape though(IMO).
And as far as ABV color, both vapes can produce dark brown evenly cooked ABV. I have 2 stems to use with my UD. One the screen is far from the heater core and I start the bowl with this one. The screen in the other stem is closer to the heating core and is used as my finishing stem. With the solo, just turn the power up to 6-7 to finish the load.
Sorry if this comparison seemed straight to the point. I typed the entire thing from my IPhone