I've got two autodarkening helmets - one a Miller XLii (I think it's called) and the other I got at Harbor Frieght, made by some company called "Arc One".
The HF one is solar powered and always ready for use. I've never been flashed by forgetting to turn it on and after a year and a half it's working fine. It keeps on being the one I grab when I go to weld. I never have to worry about it getting binged up because it was $49. on sale.
But it's max darkening number is 11, and I thought I'd sometimes want to have the full level 13 and I bought the Miller. The Miller is slightly more comfortable because it came with this nice pad for the headgear. It turns on with a little button inside and it has a little knob to adjust 'sensitivity' that I can't push or turn with gloves on, and it'll turn itself off after a while so it needs to be checked to see if it's on all the time. It does go to 13 and it seems a little bit lighter when there's no arc to look at but I think that overall it's a PITA so it stays in it's bag in a drawer. I think I don't want to mess it up because it cost around $200.
Something else about the Miller that I don't like. If you need accessories, like maybe a magnifying lens, you have to find Miller parts or be prepared to cut, file, or otherwise modify generic pieces. This is true in most Miller equipment I think - they make their stuff a little different so you have to buy their proprietary products at their usually higher prices. It means that the hardware store won't have anything to help you use a Miller product on a Sunday in a pinch.