Which external GPS should I buy?

On iOS:
Dual XGPS 160: In my opinion the best option currently. It’s 10 Hz and supports GLONASS satellites.
VBOX Sport: Great option if you have more to spend. Runs at 20 Hz. Make sure to get the external antenna.
Garmin GLO: Great accuracy and affordable, but it must be connected through iOS Bluetooth settings, and you’ll get only 2.5-3.5 Hz update rate in iOS.
Notice: Popular Bluetooth GPS receivers like Qstarz BT-Q818XT/X and similar will not work on iOS.

On Android:
Garmin GLO: In my opinion the best option currently. You’ll get 7-8 Hz update rate and supports GLONASS satellites with great accuracy.
Dual XGPS 160: Not quite as accurate as Garmin GLO, but 10 Hz and supports GLONASS satellites.
VBOX Sport: Great option if you have more to spend. Runs at 20 Hz. Make sure to get the external antenna.