TLDR - OS 2 and Wasteland 3 are probably my favorite RPG's of the last decade (since New Vegas and Dragon Age: Origins, probably), and I heartily recommend both, but these are also very different games.
I finished Wasteland 3 this morning and am in a proper place to give a comparison between it and Original Sin 2. I'm just going to make a list of different traits and detail the two games.
Quick Facts
Game Length (Main + Extra, from howlongtobeat.com)
OS 2: 97 hrs
W3: 51 hrs
Metacritic Score
OS 2: 94
W3: 85
Steam user reviews
OS 2: 95%
W3: 80%
Personal Numbers
Hours played (according to Steam)
OS2: 172
W3: 77
Game Progress
OS 2: 85%
W3: 100%
I had a game bug out near endgame of OS2, so never actually completed (though this was primarily my fault as I loaded it full of mods and at some point they must have become incompatible). I restarted after release of the Definitive Edition for another playthrough. I am familiar with the game's endings, however, and don't think the comparison will be impacted.
Personal comps
Story: OS 2
OS 2 weaves a more epic tale. However, this really may come down to personal taste. Wasteland is a little more straightforward in its core story, but the game's design is more about how you choose to tackle that story.
Graphics: OS 2
Wasteland 3 vastly improved graphics from W2 and I think the game looks great. That said, OS 2 is gorgeous and despite W3's enhancements, it still has the advantage here.
Music: Push
Both of these games have some incredible music, but they take very different approaches. OS 2 has some remarkable orchestral music. I recall a well of emotion as the "Rivellon" theme began to play as my party entered Act 2. Remarkably beautiful song that I remember listening to on repeat as I worked a few times. W3, on the other hand, has this quirky, unique soundtrack of mostly folksy western renditions of popular songs from the 80's and earlier decades. This music was incredibly well done and its placement in the game really enhanced certain encounters.
Content: OS 2
OS 2 is longer. OS 2 has more quests and a great variety of them.
Reactivity: W3
I don't think OS 2 is a slouch in this regard by any means, however it certainly feels like W3 has much more expansive branching.
Replayability: W3
I'm giving the nod to W3 here for the above reason, though at the moment my overall hours count is much higher for OS 2. One advantage OS 2 has is with companions (listed below) as your experience will vary drastically by who you select to compose your party early in the game.
Companions: OS 2
The companions are pretty cool in W3, but they don't share the depth that the companions of OS 2 have. OS 2 just has more options for ways to interact, unique quest lines, relationship-building, etc.
Characters: Push
Both games are populated with extremely memorable NPC's.
World Building: OS 2
The world of OS 2 feels more lived in and realistic. W3's various settlements don't really feel much like authentic cities, more like they are populated by essential characters and a few guards. In contrast, OS 2 cities have children playing in the streets. The OS 2 world is also jam packed with content. That said, cruising around in the Kodiak through the wastes of Colorado is still satisfying.
Setting: W3
I love the quirky Wasteland world, but definitely have an affinity for post-apocalyptic settings. Original Sin 2 is more of a traditional fantasy world, though it has substantial lore behind it and some unique traits within it.
Quest Quality: OS 2
OS 2 just has more, and more variety of oddball quests. Wasteland 3 is no slouch in this regard, however, and the aforementioned branching quests probably have slightly greater depth.
Writing: Push
OS 2 VASTLY improved its writing from the original game. Despite a more serious tone overall, it retains some of the silly humor that is a staple of Larian (I actually like this, personally). W3 is populated with oddball humor and a variety of pop culture references.
Combat: Slight advantage to OS 2
Both are turn-based. I probably prefer the speed and flavor (modern weaponry) of W3 if I'm being honest. However, I think OS 2 has some more variety, is slightly more tactical, and I found the environmental interaction to be a lot of fun.
Variety: Push
Both have extremely interesting and unique worlds/characters (as previously touched on). From a customization perspective, OS 2 probably has more unique looks and is more loot-tastic overall. However, W3 provides you a lot of options to customize your characters, modify weapons, provide unique looks. From a character-building perspective, OS 2 probably also has more overall options, but for W3, the interplay between attributes and skills is also interesting. Watching the ranger base evolve over time based on your decisions/recruits is also fun.
Polish: OS 2
Not much comparison between these two. OS 2 has the definitive edition, but even the original version felt quite more polished to me. I didn't experience anything game-breaking in W3, but did see quite a bit of funkiness.
Overall
I have to acknowledge that if I am trying to be objective, that the advantage here should go to OS 2. But this is my review, and my personal tastes prefer the W3 setting, the story reactivity, and all the pop culture references. I'm going to call this a push. If someone were to tell me they wanted to play one of the two but could only pick one at the moment, right now I would recommend for that person to pick Original Sin 2, since it is the more refined/complete product (probably cheaper now too). W3 will improve over time in this regard, however, and I hope it gets some awesome DLC content. InExile surpassed my expectations in this round, serious kudos to them!
I will work on getting a playthrough of POE 2 going next, add them to my comparison/contrast once I have put enough into it to start making judgments. POE was my favorite from the previous iterations, and I have been pleasantly surprised to hear that many of you like POE2 the most of these three. I'm looking forward to seeing how it compares!