The footpod is way more accurate, especially if you do a few runs outdoors to get it fully dialed in.
YMMV but for me the accelerometer is hilarious. The faster I put the treadmill the slower the watch says I'm going. I am not even kidding. I can set the treadmill at 6:00 mile and it will say I am doing 11:00 a mile but if I reverse it then the watch says I am doing 7:00 a mile.
Every garmin I've ever had with the accelerometer has been that way.
My old 920XT seems to be more accurate on treadmill than my new 935.