The UD is an evolution of the Old one.
I've been loving the Old one in the past (in spite of the fact it is much heavy for my thin wrist and thus uncomfortable), then the UD was presented, and I was still liking the Old one for a while.
But after maybe 6 months to a year I came to prefer the UD quite clearly.
Some may prefer the Old one because they possess it and thus have a unique link to the watch or they prefer vintage watches and the first Dato has more conservative elements (Roman numerals vs applied, 5 minutes numerals vs none, Fonts thickness etc...).
And others prefer the UD as the look has been modified to look a little purer and modern (this is the main reason why I prefer the UD over the Old one). A little less "heavy" I would say too, even if the word is too strong.
Thus, everyone will have a different opinion but I think we can say they are both answering to different needs. Especially the size as there are a lot of customers asking for bigger cases. One is not better than the other (I don't talk about longer power reserve as it is not important to me).
I really don't think one is more balanced or more pretty than the other: this is a subjective feeling, not objective and it depends on the watches one possesses or is used to handling.
Someone who likes vintage will always prefer the Old one, someone who likes more modern watches will certainly prefer the UD (aside from objective criteria as the size issue and budget of course).
However, this being said, you really should not look at what people think to make your own decision as long as your question seems to focus on Aesthetics.
I know it is more difficult to find the Old one for a try but seing both watches is quite mandatory if you want to feel you do the right move.
The UD is definitly bigger (even if 41mm is not that big either compared to a Patek 5070 for instance), if this is an issue for you. But I don't know if there is a huge difference in terms of weight.
Anyway, these are fantastic watches.
Cheers, Mark