Abu Dhabi is the second most popular spot of UAE with history from 3rd millineium BC. With great spots like, emirates palace, yas island, desert safari, zoo, mangrove park, heritage village, museum to visit, best time to travel is between April to May and September to October. On the other hand Dubai called as cosmo metro city of UAE, with history from Mesopotamia has some mesmerising spots like Burj Khalifa, museums, aquarium, fountains, malls, global village, gardens, beaches to see. From Dubai Abudhabi is 2 hours away. So I would advise you to cover both the cities, based on your travel plan. If you are planning to travel to only city, then I would suggest Dubai for there is lots to explore, but plan for abudhabi trip another time.