two questions

I can't speak to the weight difference in the glass types. But I've had Garmin Pay since Oct or Nov last year with the VA3 and since Mar of this year with 645.

You should only have to enter the PIN once per day as long as you don't take the watch off. A common scenario for me is I enter my PIN to buy coffee in the morning but don't have to enter it again when I stop for groceries (or beer) on the way home. But if it loses HR signal for any amount of time you'll have to enter it again. I have seen some posts on the VA3 thread where people were saying they had to enter their PIN multiple times per day but it's never happened to me.

I can confirm that on the F5 Plus, you do indeed only have to enter the pin once per day (or several hours; I’ve yet to discern which - a reasonably long time anyway), as long as you don’t take the watch off your wrist or break the wrist HR sensor lock on your pulse in some other way. The pay thing actually works quite well, as long as your bank supports it (in UK the Starling Bank option is dead easy to set up and I just transfer money in to that account as I need it). I did have one failure whilst paying for parking yesterday, where the parking machine and watch initially interacted (F5 Plus vibrated), but no payment was actually taken and the machine only wanted card payment after that (wouldn’t accept any contactless from card or Apple Pay either - so probably not a Garmin Pay issue).

Thanks for your replies. This solution seems like a very sensible trade-off between security and convenience. I happen to have a Santander account who are also on the (short) list of supported UK banks.

With Apple Pay, users typically tap the fingerprint reader. If no reader then a PIN. Not an apple guy so correct me if I'm wrong. So the only option for your watch is to have a PIN to enter.