So, I thought a bit about DDDA in relation to DD2 after loading it up again.
Light is more important in DDDA. In DD2 you can kind of get around without your lantern if you have to, and in misty places at night it can actually be easier to see without the glare. Additionally there are more environmental light sources and even distant light offers some small degree of visibility. Campsites are generally around if you've properly explored locations in DD2, so being out at night never felt much of a problem to me. This makes the dark much more threatening in DDDA, which I think is a big plus point.
The animations are much more fluid and look much better in DD2, however movement is more precise and the controls feel more direct in DD1, mostly because it has simpler animations. This is a problem with pretty much all newer 3rd person perspective action games; as the animations get better, they take longer to play and add dead spots where the player can't easily exit an animation that has started which leads to input delay. This doesn't effect DD2 that badly but it is still a small issue.
DD2 obviously looks a long way ahead in character models and faces, and the character creator is an incredible step forward. The lip flap animations in both games look a bit janky in most conversations.
As for vocations, I'm playing Strider at the moment in DDDA and its as fun as I remember it. Having 6 active skills instead of 4 offers a good degree of build customization and the playstyle is fun. However, DD2 having moved a lot of active skills into the vocation's core set means a Warfarer Thief/Archer has as many skills to play with as a Strider (and more if additional weapons are equipped) but a number of them are locked in. Even taking Warfarer out of the equation, I think there are a greater number of viable melee/ranged builds in DD2, but the extra slots make spellcasters more versatile in DDDA. I doubt I'll ever play Trickster for any length of time as it seems like a lot of messing around for very little benefit, but it does offer a proper illusionist playstyle for the three people in the world who are looking for that kind of experience.
Enemy variety is a big issue with DD2 compared to DDDA. However compared to DD1 before the expansion I think DD2 has the edge. At the moment I have to give this to DDDA, but after an expansion is released I hope this will equal out.
Encounters/challenge. Once you've levelled up sufficiently in DD2 (which doesn't take that long) everything in the game aside from Green Drakes and Dullahans are very easy to kill as long as you're careful with resource management and don't get dogpiled by multiple groups of enemies (especially in Battahl). Having the loss gauge only refill at rests makes the Unmoored World a much more challenging place to visit due to its mechanics and I really enjoyed exploring it and having to be really careful with my resources. For me, DDDA kept the challenge level higher for much more of the game.
World variety is greater in DD2, and the environments look much better. I miss some of the unique locations in DDDA (particularly BBI, Soulflayer Canyon, and the Witchwood). Similarly, I think I prefer the Catacombs in DDDA to any one dungeon in DD2, and Gran Soren is a more interesting place to explore than Vermont. However for me the overall variety of locations and environments in DD2 offer a superior overall experience (multiple towns/camps, a greater number of biomes, places that feel different to explore, the Unmoored World).
Combat. I massively prefer fighting big monsters with a Warrior in DD2 to anything in DDDA. It just feels so satisfying to smash huge enemies to the ground with a big sword/hammer. Due to animations DDDA had the edge on responsiveness, but I felt combat felt less spammy/repetitive in DD2 and there were more ways to deal with problems.
Sound/Music preference is quite subjective. Environmental sounds are great in both games, I prefer the music in DDDA.
NPCs: In DD2 I didn't care about Ulrika. Wilhelmina just left. The dude in the bar game me some quests. Empress' cat guard woman really liked taking baths. Gay Elf's Fairly Hot Sister's attempts at speaking English were quite endearing. Sara was a lazy, annoying stalker. Gay Elf kept trying to find excuses to give me flowers. Dwarf smith was a bro before he retired. His wife was antisocial. Sven was sort of there and I might try romancing his mum in a NG+ sometime.
Mercedes was interesting then left. Madeleine was fun. Selene was quiet. The duke's wife had a weird face. I wanted to kick the Jester off the top of the castle tower. The duke was a douchebag. Evil wizard gollum thing was there for a bit then died. The Dragonforged said some things.
The billboard quests in DDDA were terrible, but the main questline was very good. The sidequests in DD2 are more interesting than in DDDA but the main quest only really gets good towards the end. There was nothing that gave me 'lack of reactivity' whiplash in DD2 the way that getting caught in the duke's wife's tower did in DDDA.
Pawn AI seems vastly better in DD2 and Dragon's Plague gives you something else to worry about (especially if you spend a lot of time hunting Drakes like I did). My DD2 pawn was the best NPC in the game for me and felt many times more interesting than my pawn in DDDA did. I even kept rehiring a couple of pawns that were around level 60 when I surpassed them because I liked the party balance with their AI and higher level pawns I tried didn't fit the party as well.
tl;dr: Both are great games and looking at a lot of the individual parts DDDA should be better, but for me DD2 is the superior game. The interaction with pawns and the way that combat works is more entertaining and those are the most important parts of the series for me.
Tellingly, after restarting DDDA yesterday what I really want to do is play a NG+ run of DD2.