Thanks for all the replies folks. I still don't know!
I very much like the idea of the heart rate monitor / ECG function, but I don't really like the look of the [fruit] watch, and I don't really want the same nagging messages ('someone you might now just posted on Instagram', 'your parcel has been delivered', 'here's a work email on Sunday morning' and so on) appearing on the watch, the tablet, the phone and the laptop. And I know I can't be bothered with creating lots of profiles for what can tell me stuff when - my life doesn't run to a defined enough schedule for that - I use 'no notification' and 'all notifications' and nothing in-between.
So I quite like the look of the Withings ones - they do heart, brief messages, and they look nice - but if I don't want the messages at all, then I'm down to the heart monitoring and the looks. And although 'looks' are a reason to put me off something, it's not a reason to buy something - my current watch looks fine. So the function I'd get it the heart stuff. And I'm now thinking I could just get a chest strap (which are apparently more accurate than watches anyway) and surely there is an ECG app?