An unusual comparison at first glance, but bear with me. Now the 840 has 'all the buttons' it's a relevant question. So here's my answer
Garmin offers more solar charging with new Edge 540 and Edge 840 Series GPS cycling computers
Garmin Edge 840 Cycling GPS In-Depth Review
Garmin Edge 840, Compact GPS Cycling Computer with Touchscreen and Buttons, Targeted Adaptive Coaching, Advanced Navigation and More : Electronics
Bike Computers and Technology Archives - Sportive Cyclist
The New Garmin Edge 540 and 840. Does it make sense to upgrade from 530?
Garmin announces the Edge 540 & 840 series of cycling computers
Introduce the rider you are today to the one you'll be tomorrow. The Garmin Edge 540 Solar cycling computer offers targeted adaptive coaching when used with your compatible power meter and heart rate monitor. Introduce the rider you are today to the one you'll be tomorrow. The Garmin Edge 540 Solar cycling computer offers targeted adaptive coaching when used with your compatible power meter and heart rate monitor.
Garmin Edge 540 Series GPS Cycling Computer, Button Controls, Advanced Navigation Edge 540 / +Power Bank
Garmin Edge 530 Sensor Bundle, Performance GPS Cycling/Bike Computer with Mapping, Dynamic Performance Monitoring and Popularity Routing, Includes Speed and Cadence Sensor and HR Monitor : Electronics
Garmin Edge 840 Cycling GPS In-Depth Review
Garmin offers more solar charging with new Edge 540 and Edge 840 Series GPS cycling computers
Garmin Edge 540 GPS Cycling Computer, Button Controls, Advanced Navigation with Wearable4U E-Bank Bundle : Electronics
Garmin Edge 840 vs 830: What Is The Difference (And Is Solar Worth It)? - Sportive Cyclist
Garmin announces the Edge 540 & 840 series of cycling computers