Difficult to determine exactly. I compared my M430 with the V650. There is a deviation of 10% (V650 showed approx. 10% more HM). However, it has to be said that the M430 is extremely stingy with the HM (sometimes I run uphill/10% incline and it doesn't show any new HM). Conclusion: I think the V650 is more accurate than anything before. The V650 has a barometer, i.e. it must inevitably be more accurate (analogous to Suunto watches). Hope it helps to decide! Have fun!