645 music vs 245 music for a experienced runner

Actually, despite the above, I think I would stick with my 645.  Now that Garmin have replaced my unit because of the faulty barometer I've revised my view a bit. (I was previously getting crazy elevation and stair numbers as reported by others.)

I find GarminPay super-useful. I was lucky in that I already used Santander so was able to try it straight away but I found it so useful that that I binned my other accounts with Barclays and moved them to Starling (which gave me the bonus of being a better bank generally). It's great to be able to go out without a wallet now, whether coming back from a run and popping into a local shop or even just leaving my wallet at home all the time and going completely cashless and cardless. If I was in Canada I would open up an ATB account rather than go back to cards. 

Despite the suspected Bluetooth issues, I do like the metal band on the 645.  It's subjective of course but I think it looks just slightly better than either the 245 or 945. Mine is the slate version but I've changed the band to sandstone, which to my eye is slightly more chic than black. It's certainly cool enough to wear all day at business meetings and the Bluetooth connection is so reliable I now turn off the ringtone on my phone permanently, just relying on the Garmin to vibrate, which means I never get embarrassed by forgetting to turn it off for meetings. With the more recent updates to the Bluetooth in the last month or so, this seems to work much more reliably now and I can't remember the last time I had a drop out or freeze. 

I don't find the barometer that useful when running normally as living in the Thames Valley it is mostly flat.  I do, however, use the watch occasionally for bike rides and here a barometer is much more useful (now it works again) for tracking hills. I love skiing too and again that profile is a great feature to have for discussing runs at the end of the day. 

I don't think I would swap the above for the minor updates on the 245. 

Note, the battery life is the same for both in smartwatch mode. The 645 lasts 5 hours in music playback/GPS mode which is enough for most marathon runners and more than I am every likely to run (I don't think I will ever do trail running etc) and the slightly more efficient 245 only lasts one hour longer on a normal run with music.

Update - one thing that some people say is useful on the newer watches is the body battery function, which is unlikely ever to be available on the 645 due to the old-style sensor.  Although perhaps a chest strap and HRV measurement will achieve a similar thing.