For a phone based app, IPBike by I for Powell can't be beat. You can customize what the data it displays with just about infinite options. I went with Ant+ sensors, but only because I didn't want to have to buy new sensors if I went with a Garmin unit (which I did). A GPS speed and bike mounted speed should show the same average at the end of the ride (it's distance over time after all), but GPS isn't as good at figuring elevation, so your climbing and descending speeds will likely be off as will your speed when under thick tree cover. FWIW, I've had zero issues with the sensors I've got from Garmin, both the older GSC-10 and newer magnetless setups.