It really depends on how you run and what you want.
The AW (not counting the Ultra) would be just fine for marathon runners who don't use their Watch to also listen to music while running, and who don't take an enormous amount of time to run the marathon. To me, a reasonable time to complete a marathon for, say, a mid-40s male who is no longer competitive would be under 4 hours (and that's kinda slow). In those kinds of condition, using an AW is probably ok. At that slow of a pace, marking laps/segments (if you want to) with the AW is ok -- it requires you to double tap the screen which when you're running fast is very hard to do.
If you are a much faster runner and would also like to mark segments along the way, I'm not sure the AW is a great choice simply because of the way it requires segments to be marked (double tapping screen).
The Ultra surpasses all these issues. I don't think it would suffer if you use it to run a 4 hour marathon, mark segments, and listen to music.