I have owned both. I had an 81 R80RT.
k bike r bike iiii.jpg (96.75 kB . 502x324 - viewed 481 times)The R80RT was truly an amazing bike. The ergonomics were perfect for me. It was so perfectly balanced I could ride no handed and, lean only, on long radius turns for literally miles at a time.
I had to sell it to get the 02 1150 and I sold it for more than I paid for it.
The boxer engine was not smooth at low RPMS and shook the bike when lugged, but sang at 50-80mph. The tranny was unforgiving and took a while to master. Compared to the sewing machine precision of the K75's engine and gearbox, the R80's engine was clunky. The 1150's is the same way. 2 pistons mashing out brute force power when stomped on. Nothing like the steady roll on of the K75 , at any rate.
Once, after not checking the tire pressure, I bounced the R80 hard off a curb on a left turn, AND IT BOUNCED ME BACK ON THE ROAD. After that day I swore I would own nothing but a boxer.
In contrast, another time I wound it hard up the gears while touring in a rain storm and it popped out of gear between 4th and 5th at 80MPH and scared the hell out of me.
Bottom line, the low center of gravity was a complete joy, saved me once, and that's why I own the 1150.
The K75 is different. I bought it, sight unseen, and flew out to the Carolinas and rode it back to Wisconsin in 2 days of super-slab riding. It took me a while, but after the used scout fairing, I found myself riding it 80% of the time. Mostly because of the smoothness, less weight, less shifting with the expansive powerband. It is my CITYBIKE.
Now, I am training my son to ride. He is an amateur dirtbiker, so he has some riding skill, but I trust the K75's smoothness and steadiness to help him along.
Wrenching. To me it takes the same skill level to do both bikes. I have always adjusted my own valves, basic maintenance, etc. Vintage dirtbike maintenance is simplistic compared to these roadbikes. But with all the info the sites like this, you can learn to do most of the non-major stuff.
Why buy one, when you can buy two for twice the price!!