645 music vs 245 music for a experienced runner

Just my two cents:

• Some have complained that the GPS tech in the new Garmins (245, 945, etc.) is less accurate than 645/935/etc. To be fair, this kind of complaint happens every generation
• 645 has the Lactate Threshold test and 245 does not. IMO, LT is a far better absolute indicator of fitness/race performance than VO2Max estimates, meaning that I can take my LT pace and get a pretty accurate 10 km or half marathon race prediction, whereas the absolute VO2 Max number has always been useless to me. (Although the VO2Max relative trend is still pretty useful for me.
• Baro also gives you floor counting if you care about that
  • Garmin Running Power requires a baro
• If the 645 doesn't support HR on the watchface, you can always get a 3rd-party watchface from the Connect IQ store that does. There are lots of them, including a watchface that looks just like the standard 645 watchface
• The temperature sensor is pretty useless. It's just ambient temperature (on your skin) which is used to correct the baro readings. I think all Garmins have this sensor, but it's only the ones with the baro that actually show the temperature data in Garmin Connect (although there's not much you can do with it). I think most people would rather know what the air temperature/weather was like on their run.

OTOH, the 245 has the better race predictor which takes long runs and training load into account. For me, that's probably the one interesting feature that 245 has and 645 lacks. However, there's other places to get that kind of prediction (like https://runalyze.com).

If they're the same price I would prefer the 645.

Exact advantages of 645M, especially lactate threshold detection and barometr. These are the main pros of 645M.

how do you visualize the lactate threshold?

Lactate Threshold is a pace and HR. It corresponds (roughly) to the pace and HR you can sustain for 60 minutes. For most people, it's the pace that is about halfway between their 10K and 1/2 marathon pace.

Once you have your LT pace, you can plug it into various online calculators and figure out your predicted race paces. One way to indirectly do this is to plug in various 10 km race times that are a bit faster than your LT pace into the McMillan calculator, until it spits out your LT pace. At that point you'll have a pretty accurate list of all your recommended training paces and predicted race paces.

https://www.mcmillanrunning.com/

(It's probably good for everything up to, but not including marathons -- most "equivalent race predictors" fail for marathons unless you've trained optimally.)

I've a 645 and quite like the device although I've matched it to a polar HR strap to get someway accurate HR readings & a Stryd footpod to get reasonable pace/distance. 

Agree 100% that the temp sensor is a waste of time, the barometer works reasonably ok to count stairs (if you care about that) provided you don't do any running activities! That sends it into overdrive & you get ridiculous numbers like 199 stairs climbed after a mostly flat 12k run. Garmin have replaced my watch once to try and fix this but it made no difference, now they want to replace it again although not sure I'm going to bother.

A local retailer had the 645M on sale for less than the 245M after the 245 launch so read into that what  you will.

Problème is without barometer i feel like backing up my 910xt...

so you're telling me to get the 245 music? But the lack of barometer seems to be a step back of my 910XT

The barometer is of dubious worth going by what I've seen & others have reported here. Running activities on a pancake flat route have continual ups & downs resulting in jagged elevation graphs & crazy stair climbed numbers. Having said all that I like the watch (645) but I've paired it with a stryd footpod & polar H10 HRM,