Not a Watch owner. The ECG feature always struck me as a bit of a gimmick.
The 3 does a good job, from what I've seen, on notifying about odd heart behaviour. And since the Watch is a one (two?) lead device, not good for real readings (doctors/hospital use six(?) lead for quick dirty, 12 for real readings). I've been using an app, Cardiio, for a few years now: seems to be able to pick up odd heart beats, just does not give you an alert like the Watches.
As someone that has heart disease and in the family, not going to rely on a Watch to be a true test, especially when even the pros get it wrong. Case in point, was not feeling well, went to emergency, did not give full 12 lead ECG, gave a six. Nothing wrong, go home. Some months later, really not feeling well, urgent care in doctor's office does full 12 lead, and the 12th lead was off a little. Right to cardio unit at hospital across the street. Day later, 80% constriction in the Widow Maker, and a stent.