Having had an 1150GSA and now an 800, I would say that it is such a subjective choice (and not all practical) as to why you might want one or t'other.
Here's my comparisons with my bikes. I know you have a 12 so it will be different but whatever !
If you travel in a group on road and far away, then a 12 will do the high road mileages if not quicker, then perhaps a little more comfortably. If you travel alone and want to pick the small tracks and high passes or generally off road, the the 800 is a better option as it's lighter, more manageable and sprightly. I avoided places I really wanted to go because I knew if i dropped the bike or things got very narrow, I would be stuffed on my own.
The 800 returns 60 mpg or better (the recent protest rally at 40mph I was getting over 110mpg!). So with my huge old 33 l tank I was getting 250 miles range on average. My 800 gives me 240 on a tank full. Both at the same sorts of speeds. The 800 for a lot less weight and a lot more fun.
I'm not saying that everyone thinks the same as there are undoubtedly some riders (taller and way better than I) who could get the same out of their 12's as I do out of my 800 (or better) in terms of fun and performance.
There are other riders who chose the 12's because they're fat barstewards who need a bike to minimise their colossal size

There are others who just want to look all 'Adventurous' to the wider public and chose a bike most who have watched TV will recognise. Others want a bike that is smoothe and effortless to ride. The F800 isn't effortless but why ride a shire horse when you can ride a jumper ? Some here are size queens and so more cc's is to them is, well, more/better
Touring is where the 12
might be better .
IF you want to carry a heavy load and cruise at 100 all day. But then many bikes will do that well. I'm happy that I can carry two-up when i want to. And my wife insists that that is often ! But I tour alone and even though camping, travel very light compared to many on here. I'm also not someone who feels the need to go at 100+ all the time. I pick lesser travelled roads and am happy at 50-75mph. The bigger bikes really do need that speed to make them travel smoothly. The 800 doesn't.
Whislt there are a few stereotypes on here, it is evident that we are all very different and seek a different mix of what we want from motorcycling and so everyone's advice comes with their own, particualr slant.
This is a decision you need to make on your own. Perhaps remove your bling, sell your 12 and try and 800 for 6 months. You can always get another 12 if you don't like the 800. Guaranteed the 800 will not be that bad you won't want to ride it even if you decide it's not for you. Money-wise you won't lose much - if anything - if you stick with used bikes. Non of these things are irreversible.
Think about what YOU want from the bike. If you tour for two weeks of the year then perhaps the 800 will be better for the other 96% of your riding. Maybe it's all about the 'look' and image you want to project about yourself ....... they are still valid reasons for chosing a bike if not particularly practical ones !
I'm a fan of the 800 myself but I wouldn't use my reasons for liking it to dictate why you should get one. I can only explain why I chose it.
Good luck.
Dave