There are probably very few people who have done more than one company on the Mekong. That said, I have sailed the Mekong with Ama and the Irrawaddy with Avalon.
I much preferred the cabin on the Avalon; it is bigger, with a 14' wide glass wall of which 9' can be opened to the outside (with a screen to keep the bugs out). I also liked the bed that faced the window, sometimes when we were sailing I would sit up in bed reading, and glance up periodically to watch the scenery going by. The Ama cabin had the twin balconies, the real one of which just seemed to cut into the cabin and the French balcony just ended up as a small alcove not really good for anything. The Avalon ship was much smaller, a total of only 18 cabins.
The food on Ama was amazing. I'm usually a fairly picky eater (although I do enjoy Chinese food). I found that there was enough choice that I never once resorted to ordering chicken (my go to protein at home) from the always available menu; the daily choices were wonderful. The dinners on Avalon were fairly good, but definitely not in the same league as Ama; the lunch buffet on Avalon was a real drag; I rarely found anything that I really liked. One thing to note was we had only 18 passengers on Avalon; that likely affected the amount of choice available, but even with a full complement of 36 passengers I'm not sure how much more choice there would be.
The staff on both were very helpful, but I have to give a shout out to the crew on the Avalon. I had hurt a knee and so I was using a cane for excursions. In many locations we moored well below the level of the road above, and the crew had to cut stairs into the sand in each location. Whenever I left the ship, there was always someone there to give me assistance up the slope, and then again to help me back down again.