I bought the R90S new in 1975 and also the R100S in 1977. Both bikes have over 200,000 miles with no major problems. The Road King came into the picture when I decided to own at least one Harley in my life. I also use the Harley for teaching the MSF Experienced Riders Course and the Advance Riders Course. In addition, the Harley is a good bike for a lazy afternoon ride through the local national forest. The Harley loves to lope along at a comfortable 55 MPH.
I made the mistake of taking the K1600 out for a demo ride that turned into a four hour ride and then ownership. What an impressive bike but having said that, there is something to a basic bike such as the R1200R or an Air Head. I am very fortunate to have an understanding wife and the ability to own more than one motorcycle. But if I had to give up all but one, the struggle would be between the R100S or the R90S. Most likely it would be the R90S as she was the first BMW in my life replacing a cantankerous 1969 Triumph Bonneville.