On the spec sheet the watches are identical. The quatix 8 is the marine version of the fenix.

Short answer

quatix 8 - marine features: boat control, autopilot, nautical charts. fenix 8 - the same hardware for much less.

Key differences

SpecGarmin fenix 8 47mm AMOLEDGarmin quatix 8 47mm AMOLED
Display1.4", AMOLED, 454×4541.4", AMOLED, 454×454
Memory32 GB32 GB
Battery lifeup to 16 daysup to 16 days
ConnectivityBluetooth, ANT+, Wi-FiBluetooth, ANT+, Wi-Fi
NavigationGPS, GLONASS, Galileo, QZSS, BeiDouGPS, GLONASS, Galileo, QZSS, BeiDou
NFCyesyes
Sensorsheart rate, barometer, compass, gyroscope, temperatureheart rate, barometer, compass, gyroscope, temperature
Protection10 ATM10 ATM
CompatibilityiPhone, AndroidiPhone, Android
Dimensions, weight47×47×13.8 mm47×47×13.8 mm, 73 g
Pricecurrent pricescurrent prices

Specs come from the device pages on Smartphone.ua.

What's the same

1.4" AMOLED 454×454, 32 GB, up to 16 days, Wi-Fi, NFC, 10 ATM, 47×47×13.8 mm.

Marine features

The quatix connects to compatible boats' onboard electronics.

Who it's for

If you don't have a boat, the quatix is overpaying.

Verdict: who should buy which

  • Sailors and anglers - quatix 8.
  • Everyone else - fenix 8.

FAQ

Is battery life the same?
Yes, up to 16 days.

How heavy is the quatix?
73 g.

Which is cheaper?
The fenix 8.

Read also