Wich one do you trust more?
Lets say the game in making is the same.
Demo team trows out a free demo and you try it. (remember, demo only has araund 10-30 min of gameplay usualy)
A good example would be a neverhood demo:
Early access ask you for the money (99% of time) to try the unfinished game. They kinda use you as a free tester. Cough cough, warhammer (and not even playeable , just a promise).
Bonus for demo:
https://www.youtube.com/watch?v=7QM6LoaqEnY
So in the end...wich game do you prefer buying?