I've used Sigma for 20 years, never been disappointed. It's as accurate as you set it. I try to allow for wiggle, that I do lots of. So the mile is the road distance within 20 or 30 feet. There used to be places on the highway to set your odo, but then I need to convert km to miles I still only use.
Never once deviated from interference. I have zero faith in pure GPS to be anywhere near. My 14.12 altimeter is just a toy too tho.
One day I used free Map my Ride on my home computer, mostly dead straight highway. I dotted out a route I had ridden. MMR kept saying something 7 miles off, like it was programmed to some other route. WTH.
Bontrager only records one decimal, useless.