I've not sat on an 1150 GS but the seats are wider, hence the R may seem lower? From memory looking at cycle ergo website, the seat height and footpegs differ on differerent year R's. Another option is the R as its cheaper. 2 inch risers on bars(no need to move any cabling, and 1150GS foot rests lower that by another 2 inch or so, so its then a mini GS...I took my centre stand off as it wasn't allowing enough ground clearance, but on a plus side its extra weight lost. Some re routing of the brake line to the slave cylinder. Runs and rides like an adventure bike for me. Much cheaper than the original GS's. Its an option
The R1150R and R1100R are somewhat different (tank/seat/ etc etc) so from an ergonomics point of view need to be assessed separately.
Also - regarding seat heights - the R1150R/Rockster came with 3 different OE seats (Low/High, and later Tall) - depending on which global market the bike was sold in, it might have come originally with a different seat, and buyers were also able (if informed) to choose a preferred seat height*. I would assume that the R850R based on the R1150R would have the same options, (the 850 was always a 'sleeved-dwon' version of the big bike, so the early R850R is based on the R1100R, and the later R850R is based on the R1150R - not confusing at all).
These are the R1150R seat options.
https://www.ukgser.com/forums/attachment.php?attachmentid=544566&stc=1&d=1661165706
* Although - I bought 2 Rocksters brand new and don't remember being told about different seats - mind you - I never asked!
