Which external GPS should I buy?

On iOS:

Qstarz BL-1000GT: This is Qstarz’s first receiver with iOS support. It’s very accurate and runs at 10 Hz. You also might consider the cheaper model without the memory card slot, BL-818GT (not to confuse with other devices with almost similar model name).

Dual XGPS 160: Great accuracy with GLONASS support and it’s 10 Hz.

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:

Qstarz BL-1000GT: Works great with Android too. It’s very accurate and runs at 10 Hz. More expensive than Garmin GLO though. You also might consider the cheaper model without the memory card slot, BL-818GT (not to confuse with other devices with almost similar model name).

Garmin GLO and GLO 2: 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.