I have the F800 GS Adventure, I bought it new, the upside of the Adventure over the standard F800 is what you get for the additional few bucks, bigger gas tank, wind screen, stronger sub frame and LED Lights and prepped for rear hard bags . Only down side I see is it is wider and heaver than the standard 800 GS. One other negative is that when you drop it in the Michigan sand compared to a 1100/1200 is that the boxer motor keeps it from falling parallel to the ground with those outrigger cylinders on the 1200 . Service of the chain does not bother me on bit, just part of bike maintainence. Also with a chain you have the flexibility to “swap” ratios with a simple change of the Counter Shaft Gear. The F800 is narrower than the 1200 in the woods if you are ever on a single track.
I geared mine down my one tooth on the Counter Shaft Spocket, so if I were to ride out West I would gear up for slabbing it, then gear down for off roading, not an option on the 1200. A 20 minute job at max.
The F800 is a lot lower price also than the 1200, so that was a key driver for me.
Last point for what it is worth, I go to Spartanburg SC for business and I tend to stop in the BMW Motorcycle Store that services the bikes for the BMW riding school. One guy at the store is one of the instructors and perhaps it was because I own a F800, he stated, overall he prefers the F800 to the 1200.
Both are great bikes, just too many choices