Hm.. I love both games. Having read all witcher books - I absolutely love the Witcher universe and charachters.
To be fair, I'm pretty new to Dragon Age universe so please keep that in mind, when I say that to me, without a doubt, Witcher 3 is a better game. In fact it might as well be the best game I ever played.
But, there are things which make me to come back to DAI long after I completed the game..
Let's see..
Graphics: Even.
Witcher 3 looks much better, sure, but it is also much heavier on the eyes and sometimes I have trouble playing it because of an eye strain in causes, particularly in some areas.
Story: Witcher 3
Main villain was also a bit underdeveloped, but to me, Witcher 3 story has been mindblowing.
Please keep in mind that I played and completed previous Witcher games and read the books, this of course makes me more invested in its world.
Companions: DAI
There were some truly unforgettable exchanges between characters, their reasonings and beliefs felt real and grounded. I loved Cassandra, Dorian and Varric. They truly felt like friends to me. Geralt on the other hand does not really have companions - and his love interests either want to use him or have used him in the past.
Gameplay:. Even
Even though I love The combat in witcher 3, how adaptive, precise and dangerous it is - after playing the game for a while it grew a bit stale. All you do is same old sword attacks and some signs. Although interesting, they are not varied enough. On their hand, Inquisition has five unique gameplay classes, and many spells and abilities so it's hard to get bored for me.
Monsters: Even.
Witcher 3 has Gryphons, Wyverns, Trolls, Leshiys - you name it. Their design is outstanding and you really have to think how to kill some of those. DAI bestiary is lacking.. Where are all those legendary monsters? Why Griffins only ever mentioned in some book?.. But then...wait for it.. Yes, right.
Inquisition has DRAGONS. And they are just fantastically amazing

Levelling: DAI.
This is without a doubt. Witcher 3 levelling design left me completely shocked - what a way to make a skill tree look imposing and big! Lol.
First of all, you can only equip 12 skills total. Which makes creation of Geralt you want to play mostly impossible. Want to use both Signs and advanced Sword skills? Tough luck.
May be some people enjoy switching skills before battle, but I surely don't. Especially when there is no simple way to do this. And finally, most of the skills are just some passive garbage. Plus something to something! And of course you also have to equip it to enable! Great.
And what about those awesome looking skills - the Whirl especially? You would think each trees has something special like that? Think again..
On other hand I just love Inquisition skill system - apart from 8 skill rule of course -passives that do stuff, active that vary gameplay and specialisations too!
Character: Inquisition.
And here is where I will conclude my long post. Although Witcher 3 is extremely fun to play - I don't see any value at all in replaying the game again, and doubt I ever will do so. In Inquisition on the other hand, I'm already having fun with other race/class combos, and what is important - you character feels like your own - you decide his responses on every step. How annoying was it in Witcher when Geralt suddenly decided to pick some random angry replies without any input on my side!
So, to sum it all up:
Both games are great. Witcher 3 to me is a better game, due to its alive world, amazing story and script, great music and atmosphere in general, and amazing quests.
Dragon age Inquisition has better gameplay, character levelling and progression, dialogue choices and conpanions, and much better longevity.