AI vs UA
If you are traveling in Economy then each and every airline of any country is going to be uncomfortable for such a long journey.
Having said that both UA and AII are good options when everything goes smoothly.
If you are vegeterian or person from East Asia or South Asia, chances are that you are usually going to be treated much more reasonablly in AI than UA. (though not always)
But biggest problem with UA is that they do not have any spare equpment in India (in fact to entire sub-continent, they have only 2 flights per day). So if something goes wrong, AI might be (equipment logistics point of view) in better shape than UA to handle it.