Forerunner 645 Music or Fenix 5 again?

If I were you I'd bought 5 plus. I love the music, nfc and long battery life (and that's something 645 don't have but 5plus yes)

Thanks a lot, yes battery life is important for me so I will go to F5 plus

As an owner of both I would go for the 645 music. My fist Garmin device coming from a Tomtom Spark was the Fenix 5x Plus I purchased this year. It is a great watch but I found myself not using the headline apps such as mapping which is just too small and clunky to be anything worth using. I returned it after 2 weeks for a full refund and got the 645 music for 1 third of the price. It is lighter, has the same screen size without all the extra plastic and has most of the same functions as the Fenix, nothing I have missed. You can buy three of these watches for the price of the Fenix 5 x plus.
if you can it might be worth waiting for the Fenix 6 which is bound to be released first half of 2019.

After 1 1/2 year and more than 7750km with the "old" fenix 5x on my wrist I got myself a 645M.
I used only about 10% of the fenix capabilities as I am a pure runner.
The 645 makes it comfortable to charge during the run so it will be my new running gear even for longer ultra runs.
My fenix 5x is going to be sold.

I have both and the only real reason for the F5+ for me is that it will pair and record power data when I bike. Other than that the 645 is all that a runner needs IMO.

After 1 1/2 year and more than 7750km with the "old" fenix 5x on my wrist I got myself a 645M.
I used only about 10% of the fenix capabilities as I am a pure runner.
The 645 makes it comfortable to charge during the run so it will be my new running gear even for longer ultra runs.
My fenix 5x is going to be sold.

I love my 645M. How do you charge it during the run (a simple external battery like those used for mobile telephones) ? Does this not stop the 'run' activity on the watch if the watch is charging at the same time ?

You are right, DavidBrux . I set the 645M to "Garmin" as USB-Mode (Settings, Systems, USB-Mode) and then I charge the watch with an external power bank and the original clip.
Works really well.
For the longer ones I start the run with the battery pack connected and I will remove it during the run when I know the watch will last till the end. It is much easier to disconnect the cable after 50km or more than attaching it. Especially when it's cold and you need to wear gloves. :)

So after lot of decisions I bought F5X Plus and I hope it willI serve me well :-)
However thanks all for your opinions and recommends

Comparing the 645 to the Fenix 5 Plus is like comparing grapes and apples. The choice depends on which one fits your wrist. Guys with small wrists or guys who want the lightest watch possible for running will get on with the 645; guys with larger wrists will prefer the larger Fenix 5 Plus. The watches are very similar functionally speaking, but the Fenix has longer battery life. Personally I don’t think the 645 battery life is poor as I don’t run ultra marathons. I’d rather charge it more often and have a lighter, smaller watch.

I have thin wrists, but would prefer the 645 in any case because IMO it's easier to use as an all-day watch, fits comfortably under long sleeves, and looks nice without being too obvious. I'm also fairly impressed with the battery life, but I don't wear it 24/7 and usually power it off from evenings to the next morning. So far I've used it for outdoors and treadmill running and indoor cycling, although I still use my Edge 1000 as the "official" ride recorder. I might use the 645 in a pinch for cycling, but it's limited in functionality and not as useful as the Edge with its 8 data fields always there in front of me.