I think you nailed it when you said the accuracy of the One varies where you wear it on your body. I have a One as well and I did an non-scientific experiemt the other day to measure the step count between the AW and the One. I walked 100 steps and my AW counted 99 steps. I then walked another 100 steps and my One counted 110. Over the course of a 10,000-step day, this will be over a 1,000 step variance between the two devices. That's when I'm keeping the One in my shirt pocket. When I wear the One in my pants pocket, the variance can me up to 3,000 steps between the AW and the One.I wore my old Fitbit for about 3 months alongside my Apple Watch, and for a 10,000 step day the two were always very close... usually within a few hundred steps. I have a Fitbit One though, which is worn clipped to your clothes or in a pocket, so it's possible that accuracy will vary depending on where you wear it. Fitbit's real-time step counting made me think it was very accurate. I would count the steps as I walked and stop to check and it was always very close. Of course stride length varies, as does effort required, with changes in terrain and steepness. One way the Fitbit got around this was to offer the stairs climbed metric. The iPhone's Health app offers this too, but only when you carry your iPhone. If you climb stairs wearing only the Apple Watch it will track your steps and your heart rate, so at least you might get some exercise and move credit for the effort.
My problem with the Fitbit is that I get a lot of my exercise commuting and running errands by bicycle, so there would be days when I would get a lot of exercise but Fitbit would not accurately reflect that without me starting a workout every time I hopped on my bike. I feel like the Apple Watch gives me a more consistent historical report on my activity and exercise levels whether I biked more or walked more on a particular day. Also, when I did start a workout, my Fitbit was terrible for tracking bicycle rides. The Fitbits that have GPS and HR are probably much better. With the Apple Watch it becomes a combination of the watch's HR sensor and the iPhone's GPS.
So the Fitbit focus on steps wasn't really ideal for me, but it's an easy metric to allow for competition within the Fitbit social network. I feel like the three rings on the Apple Watch are better at reflecting how active I was on a given day.
Sean
I think that between the two, the AW is more consistent in measuring steps and exercise minutes. The Fitbit can vary wildly on exercise minutes day to day.
IMO, I think it's better to set goals based on active calories burned using the AW vs. steps on the Fitbit. I'll still use my Fitbit until it dies, but after that, I'll probably use the AW as my primary fitness tracker.