The topclimber and ATN climber are very similar, but different.
The top climber the halyard slides thru the ascenders and the atn climber they can fit over the halyard. With the ATN there is no need for a separate line. You can climb your halyard.
They are both very safe and have been dropping in price i see. I prefer the ATN climber even though the ascenders are not as well made as real ascenders from REI.
The bosuns chair would be very difficult to fall out of if used as instructed and is far more comfortable than a harness. It does have leg straps. If you fall out of the chair than you have no business being up a mast.
I used to use the harness for hunting; no go. Now with the ATN climber i can feel my toes again. You can hang out for hours. I bow hunt from it.
It does take some time to really learn how to go up and done with the ATN in a rapid fashion. I'm quite fast, but i use a swinging type climb rather than the one you see in the videos. Its actually easier on your knees and waist. I swing my feet up(out) as i pull up the ascender; then push down. Not the best move if it is rough out though.
Compared to putting your on kit together it's roughly the same price.
Ascenders~ $80 ea
Harness~ $40-50 (if not more)
Different strokes for different folks, I suppose. Maybe body shapes are different. I bought a bosun chair and felt very insecure in it, no comparison to the very secure feeling in the properly adjusted harness. And yes, mine is supposed to be a good chair, it was the top model of BoatUS (the last year before they got absorbed by WestMarine). I agree that the harness does not quite have the comfort level of a sofa though.
I looked at the ATN "ascenders" somewhere (maybe the Annapolis boat show) and found them to be rather Mickey-Mousish. The thingies they call ascenders sure are no Petzls... And they asked a lot more money for it than for proper equipment so it was a no-brainer for me. You say prices have come down on it, so perhaps it is different now. It would have to be a lot cheaper than proper rockclimbing ascenders to make it interesting. For me, I never looked again after I bought my equipment (10 years ago).