Another difference is the CPU, which is really bad on 645 (and all the others). If you play music from it it dries up the battery pretty fast. It goes nowhere near 2 weeks, as you say, for me at least. I’m keeping all the features active, though (bluetooth, gps, glonass, garmin pod, heart rate monitor).

Feature wise I honestly think that Garmin makes better sport watches, while Apple does better smart watches.