Fenix 6XPro or 6X Pro Solar

Solar does not charge at all. It reduces the power consumption 

No, it is not fully charged. Only some 2-4 % in 6-10 hrs. Tested that. An additional foldable solar panel is more effective. Got 4% charging in 25 minutes. As said before, it is more for reducing power consumption.

For unknown reasons the availability of the solar model is worldwide very limited at the moment

IMO I’d only pay the extra for the solar if your normal average weather is good enough. I live in the U.K. and decided against the Solar version as didn’t think it would add enough benefit.

The specs says:

60 hrs with gps +6 hrs solar (under best sunny conditions). So the maximum solar charging rate is 6 hrs additional gps use in 2,5 days (about 10% battery). Or I missunderstood something.

I think that's correct. I also believe I saw that the Garmin specs assume something like 3 or 4 hours of sunlight exposure per day. So if you get more (or less) sunlight in a day, you'll get more (or less) additional battery life.

At any rate, you can expect your battery to last about 10% longer with the solar model, versus normal 6X. You have to decide whether or not that extra 10% battery life is worth $150. To some people it is, to some people it isn't. For myself, it wasn't worth it, because the 6X battery already lasts WAAAY longer than any of my activities, and in the off-chance that I ever need a little more battery life, it only takes an hour to get a dead watch battery back to 100% with a portable battery charger. In the time it takes to make and eat a sandwich at the summit, I can recharge the battery and get another 60 hours of GPS time out of it. Sure, I have to carry a portable battery charger, but I do that anyways since my watch isn't my only rechargeable electronic I have with me.

The only scenario I can see the solar being a huge benefit, are the people who participate in 60-66 hour events, where they have no other way to recharge the watch during the event. If the event is shorter than 60 hours then the regular 6X battery is enough to carry you through, and if the event is longer than 66 hours then your battery is going to die before you're finished regardless of whether it has solar or not. There's a magical window between 60-66 hours where solar could make all the difference.

So if you see yourself doing events that last precisely between 60 and 66 hours in duration, where you can't charge your watch at any point during it, then solar is definitely a good choice. Otherwise, it's not likely to make any difference.

That said, if $150 is no big deal for you, then perhaps you might as well go with the solar option - but since you posted this thread, it sounds like you're more like myself, and would rather save the $150 if there isn't much real-world benefit to having solar. Also keep in mind that there is no Sapphire option for the solar model, which means the glass is going to scratch easier so you'll probably need to use a screen protector.

Unless you’re in an area that gets a lot of sun like the South Western US or are outside a lot I would say the Solar is not worth it. I only got the Solar because I am in the South Western US. It does work well while hiking and like others have said it’s not used to charge the device but to offset battery usage.

There seems some availability issues with the Solar unit. We were said it'll be here by November. However, now it's said it had some problems, so Garmin withdraw the batch and consumers should wait one more month...

Why are you saying that the battery will die if the event is longer than 40 hours? The specs says 60hrs with GPS on the Fenix 6X-versions.

Well, for me was the weight (and looks) a more important reason to go for the solar version while the solar option is a nice to have. My previous model was a Sapphire version and I do like the power glass more (more clear and less reflective), but that's also a matter of taste. 

I was told that the supplier of the power glass has manufactering issues...