I have to agree what sayed here before - the 1200GS loses some ground in comfort (dunno whats got into BMW heads lately(?)), sayed by me and by my better half we mostly ride together (she's nearing 100,000km as a pillon) the R11xxGS (R1100, 1150 and 850GS have the same stock seats) has noticably better passanger comfort. After my two hours of R1200GS test drive i also find the rider seat is a bit more uncomfty, not losing much tho. Well, 2 hours isn't much but it looks like if going for 1200 consider some aftermarket seat for your passanger, and maybe for you too, if planning to do a long legs on travels.

We've been on V-Strom, Affy Twin, Capo Nord so far the ancient old GS has the best on comfort for both. But indeed, don't give up the others just because of that: you may get aftermarket seats for any of the bikes. The footpeg and handlebar configuration is your next problem on the rest of the bikes, indeed. But with lot of playing with different configurations you may find ideal positioning for you and your pillon. Had to do this with all our previous bikes, but on the GS it was just "sit on" as it is and continue to ride, nothing starts to hurt. We're mostly doing 600 to 1000kms per day on travels, meaning full day of intense riding on shaky roads mostly.

Re top box as a back rest for pillon: don't count on it! Driving fast on potholed asian roads the normal plastic TB vibrates so lot, that it rather kicks your passanger's back than supports it! We dropped the TB idea soon enough after going outside Europe, elastic waterproof luggage roll is the thing for us.

Looking at the pictures Grant had much different (own built(?)) top box compared with the "regular" ones you can get from the shop, that fixes a lot more onto bike than a single top-box-frame, i think that's why Grant recommends it - it doesn't vibrate that much. I've had lot of experiences with the "regular" top boxes - they are good things to have on a smooth roads of europe - you can secure helmets in them etc, but after 3 times broken rear frame on potholed roads - not that good idea in third world road conditions! Maybe there are some smart TOP box mounting systems around that fixes those vibration issues, but i haven't seen one yet.

Those special (mostly made only in USA) backrests - everybody have different views about it. Altough it seems to be the best option for passanger's back resting we don't use it for safety reasons and it proved to be the right one after high speed crash when the bike rolled over us few times, don't want to know what would have been happened if there was a chopper-style thick metal framed back rest installed that doesn't bend even with huge force and the bike lands with full weight on it, you between and your passanger's spinebone in contact with it? Food for thought...