Weight vs Forerunner

I came from a 935 to a 5+ and then a Marq. Initially I found the 5+ heavy in comparison but soon got used to the extra heft.  Now I don’t even notice the difference. 

I tried the fenix 5s a long time ago and I also saw it uncomfortable. I thought the 6s being shorter would be more comfortable. I was able to try the regular 6 and it was heavy, but more comfortable than the 5s. I still haven't been able to try the 6s and I'm afraid to buy it and make it uncomfortable like the 5s.

I have a 645 and found the Fenix 6s to be noticeably heavier when I first picked it up but after wearing it a few days I either don't notice or care. I don't like to wear my watches snug so I don't know about that. 

I came from a 935 that I loved.  I got a Fenix 5+ and it was never very comfortable.  After 9 months, I never quite got used to it.  With the F6, I'm finding it much more comfortable than the 5+ even though it's only 4 or 5 grams lighter.  The shape of the bottom is similar, but there are slight differences.  It may not work for everyone, but don't let the discomfort of the F5+ scare you away.

I have a pretty solid 6x sized tan line on my wrist. I think I'm stuck with those size as I'm pretty sure my wrist has adapted. it actually isn't fully covered when I wear the MARQ

I come from a 935 -> 945 -> 6s with small wrists.

The 6s feels a bit bulky at first (same height as the 935 and 945, but noticeably smaller Diameter). Didn't get used to it the first day and thought of returning it.

But the 2nd and 3rd day it feels much more comfortable than the 935 and 945 especially while riding, running or Swimming. Maybe I had been paying a lot more attention on that issue at first... 

The 935 and 945 were both noticeable while riding, the 6s isn't.

The 12g more weight (~20%) are noticeable while running or Swimming at first (maybe because of the additional attention payed on that issue), but after about 50m Swimming and the first 500m running that Feeling was gone and it felt just like the 935 or 945 weightwise. The reduced Diameter allows me more Flex in the wrist, that lets me absolutely forget I wear the watch

Ultimately you can't have it all, a Fenix 6 is designed for durability and outdoor activities (and it does a great job at it) and I'd say a Fenix 6 also has a more attractive look and might be a more suitable watch to wear to work, but a fenix 6 is not a runner's watch. 

Sure you can run with it, but you can't expect the same comfort (lightness is important while running) and faster instant pace changes of the forerunner (a plastic advantage). The instant pace issue is something a footpod from garmin or stryd can resolve. (You have to decide whether this is an option for you, for me personally while I do have a footpod I rarely use it as I don't like external accessories, especially not when I have to switch them from shoe to shoe (I have a lot of 'active' shoes I use in rotation)).

For the importance of the weight that's something you have to decide and is a hard thing to get advice from someone else. Maybe you can visit a runner's store where you can actually wear the watch? A run of a few minutes on the treadmill should already give you an idea...

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That's true but I really do think the 6 being slightly lighter than the 5+ is fine for running. Unless you are doing Ultras, you won't notice it. That's my view anyway as I have just moved from a 945 to a 6.

"you can't expect faster instant pace changes of the forerunner (a plastic advantage)" - can you care to elaborate on that? Are you suggesting that the GPS antenna strength is compromised by the metal casing? I thought they sorted that problem out after the Fenix 5.