With the annual motorcycle inspections, I spent an afternoon shuttling each of my bikes to the inspection shop, so I got to do a lot of back to back riding. The weight is definitely more noticeable on the R18 vs my GSA which is 250lbs lighter, but my S1000R is still 100lbs lighter than the GSA, so it was easier still. The Vespa is 100 pounds lighter than the S1000R, so feels almost like a bicycle in comparison to the R18C given it is 455lbs lighter.
This past weekend I took the R18C up the Connecticut coastline for a leisurely 150 mile ride to grab a lobster roll on Saturday. The way up was all back roads & surface streets as I tried to hug the coastline as closely as possible, so lots of stop & go. The way back was all highway. I do think the R18 is better for back roads or highway than surface streets with lots of stop & go. I took my GSA out on Sunday for a 200 mile day ride to see a motorcycle museum in upstate CT with some similar back roads & surface streets. Maybe its because I was used to commuting into NYC with the GSA, but I felt I could stop at any stop sign & not need to put a foot down, it is so well balanced and has a steeper fork than the R18 allowing you to balance at a stop more easily. The R18 definitely needed a foot down. My Vespa is my around town vehicle of choice with lots of storage space for errands. So practical minds will lend themselves toward the GSA or Vespa for around town/commuting duties.
But the R18C pulls the heart strings more than any of my other bikes. It's the bike I'm most likely to pull out of the garage when I just want to go for a ride & represents a bit over 30% of all miles on my 5 bikes in the past 3 months & it's still been cold in the Northeast for much of that time.
The RT the OP switched to is a great all-around bike. I like taking dirt roads (and some single track when I'm really in the mood), so if I could only have one bike, it'd be my GSA no question as it is the most versatile in my fleet. If one doesn't do any dirt, the RT is a solid choice as an only bike too.