For me, DA:I wins on:
Main Quest I found Inquisition's main quest had more of a sense of purpose and direction. I also enjoyed going to all the special quest areas like Therinfall Redoubt and how unique each of the main quests felt. I think TW3's main quests were better integrated with the rest of the game however.
Characters Both games do this really well, but characters in Inquisition feel more fleshed out overall.
Exploration TW3 has forests, some good urban environments and Skellige. But Inquisition wins on sheer quantity and variety of unique locations. True, its urban environments are lacking (Val Royeaux was gorgeous, but small) but it makes up for it in different ways - for example by having many of the main quests take place in unique enviornments.
Choices Again, both games do this really well, but compare Bloody Baron in TW3 In Hushed Whispers/Champions of the Just in Inquisition. In Bloody Baron you get two different endings. In DA:I's quest, you go to one of two unique areas and get four different endings. That's just an example, but it feels broadly representative of the difference between the games. Dialogue choices in Inquisition also offer way more options than in TW3 (such as when Josephine asks about the Inquisitor's background when in Haven).
Customisation OK, the armour variety for Qunari isn't great (dwarves and elves are fine, humans get a good amount), but in TW3 all the armour is for just one character model. Both games are probably equal when it comes to armour choices for the main character(s) once you've taken into account the greater number of possible player character models in Inquisition. Armour aside, we can also choose the player character's facial appearance, race and gender in Inquisition. And I have to admit I prefer Inquisition's hair/horn styles. Geralt has far fewer hairstyles available to him and only the French ponytail and loose long hair look good to me. On top of that, in Inquisition you can spend hours messing with crafting and tinting armor for your companions, each of whom gets a unique look for each piece of armor. Finally, you can decorate Skyhold and choose from a large number of different mounts in Inquisition.
Combat Inquisition has a greater number of ways the player can customise their combat experience - class, specialisation, party set-up, tactical cam on/off - which leaves it feeling a lot more varied and interesting.
For me, Witcher 3 wins on:
Side quests I don't really need to explain this one. TW3's side quests blow DA:I's side quests out of the water.
Creating a living breathing world While the environments might not be as varied as those in Inquisition, they certainly feel much more alive since they are populated so heavily with all sorts of curious people who have engaging stories that Geralt can interact with.
Side activities TW3 and Inquisition are mostly equal in this department - TW3 has monster nests, DA:I has Fade Rifts: TW3 has hidden treasure, Inquisition has shard collections; TW3 has bandit camps; DA:I has requisitions - but TW3 wins because it also has Gwent and Gwent is amazing.
Feeling of agency This is a weird one, since despite Inquisition having more choices, TW3 provided a more consistent feeling of agency overall. This is tied in with Inquisition keeping significant choices largely confined to interactions with characters in Haven/Skyhold and during the main quests. Not being able to make those kinds of choices in explorable areas led to long stretches of the game where the player character doesn't have any meaningful sense of agency. In TW3, Geralt is faced with choices on a more consistent basis, which led to a stronger feeling of agency for the character on the whole.
I'd say both games are tied on overall fun factor/addictiveness, but on the whole I personally prefer Inquisition. it seems that what this comparison mainly boils down to when it comes to Inquisition is that quest content doesn't feel as integrated into the explorable areas as it ideally should be.