Changes over the M400 from DCR:

Added optical HR sensor: Polar believes this 6-LED sensor is their most accurate to date
Added vibration capability (alerts): This does however replace audio alerts, which go away
Slightly increased battery: Mostly to maintain battery life with added optical HR sensor
Added new low-power GPS modes: These enable the unit to get up to 30 hours of GPS-on battery time oHigh Accuracy Mode: 1-Second Recording Mode: Plots a GPS point every second, HR is every second too oMedium Accuracy Recording Mode: Plots a GPS point every 30 seconds, HR is every second still oLow Power Recording Mode: Plots a GPS point every 60 seconds, HR is every second still
Changed the wrist strap design: This was to improve optical HR accuracy by reducing weight and increasing tightness
Added new watch faces: These can be changed in the menus to float your boat
Enabled Fitness Test with optical HR sensor: This is pretty rare in the industry
Added new sleep algorithms: This will give additional data in the Polar Flow app
Firmware Updates Available via Bluetooth Smart: This unit needs no desktop computer at all
Added Stopwatch functionality: Pretty straightforward I think
New connector: This new connector replaces the micro-USB used previously that was a support nightmare
24×7 HR coming this Q3 2017: Currently the unit doesn’t record 24×7 HR, rather, only workout HR. A firmware update later this year will address that
Redesigned charging cable provides more reliable waterproofing

Polar M430 Running Watch - Unisex