I was kindly loaned a Magellan 105HC from the supplier for the Wombat 100 MTB race earlier in the year, so set it up alongside my Bryton Rider 50 for comparison. Neither unit was paired to a wheelspeed sensor, relying solely on GPS reception for speed & distance recording. Both units were paired to the same cadence and HR sensors. Not surprisingly, these two fields were reported consistently across both computers. However there was a substantial discrepancy between the distance recordings. I'm fairly confident in the reliability of the Bryton's measurements, having cross-checked it against various other non-GPS devices, so it provided a good benchmark. The Magellan constantly read a slower speed, at times by more than 10 km/h which is a big discrepancy given the lower speeds of off-road riding, and correspondingly less distance was recorded, 93km vs 100.7km on the Bryton. More curiously, its timing was also slower. Not sure how that works.
I don't know what goes on in the guts of these things, but I'd hazard a guess that the processing brainpower in the Magellan isn't as sharp as the Bryton's, because as well as the lower displayed speed & distance, its response to variations in speed wasn't as sharp.
Given Magellan's long history in the consumer GPS market (along with Garmin they're the pioneers of it), overall I was a tad disappointed by it. The device is easy enough to use and set up, but the accuracy of it left a bit to be desired.