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.
Qstarz BL-1000GT: This is Qstarz’s first receiver with iOS support. It’s very accurate and runs at 10 Hz. More expensive than XGPS 160 though.
VBOX Sport: Great option if you have more to spend. Runs at 20 Hz. Make sure to get the external antenna.
Garmin GLO and GLO 2: Great accuracy and affordable, but it must be connected through iOS Bluetooth settings, and you’ll get only 3-4 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 and GLO 2: In my opinion the best option currently. You’ll get 7-8 Hz update rate and supports GLONASS satellites with great accuracy. The only difference between GLO and GLO 2 seems to be that the GLO 2 keeps connection more reliably with new Samsung phones. Notice: You will need to upgrade GLO 2 to firmware 2.10 or later, using the Garmin WebUpdater on Mac or Windows, otherwise it will not connect with most Android phones.
Dual XGPS 160: Not quite as accurate as Garmin GLO, but its 10 Hz and supports GLONASS satellites.
Qstarz BL-1000GT: Works great with Android too. It’s very accurate and runs at 10 Hz. More expensive than Garmin GLO though.
VBOX Sport: Great option if you have more to spend. Runs at 20 Hz. Make sure to get the external antenna.