My first flight during COVID to Canada was with Etihad, headquartered in Abu Dhabi, and enjoyed the experience (+ very attractive fares at the time) so much that I decided to fly with them again for this next trip to the UAE.
Having never actually visited Abu Dhabi before, I thought it would be a good idea to start and finish my trip in this Emirate, which essentially is the political, cultural and financial capital of the UAE.
It was far quieter than Dubai, and far less 'glitzy', albeit far from 'modest' given the many examples of extravagance on display. I had a chance to catch up with a number of friends who had preferred to reside here over Dubai and not only for business reasons.
The Sheikh Zayed Grand Mosque is the largest and most beautiful I've seen in my life to date, and I can say the same for the Qasr Al-Watan, as a presidential palace. The attention to detail was outstanding. I enjoyed the architecture of the Abu Dhabi Louvre more than the actual exhibits.
My friends in Dubai complain about the lack of diversity and international options for food and entertainment, but I did not feel that way when I visited - I even found fusion Californian/Afghani cuisine which I quite enjoyed.
Overall, in terms of first impressions (having not spent nearly enough time here to properly judge) I actually prefer Abu Dhabi to Dubai, but that is a very subjective and personal preference.