I have the Transcontinental now, and my previous bike was the Grand America. One of the reasons I sold the Grand America was for its instability on the freeway when riding solo. Nothing I couldn't handle, but on long freeway drones it became fatiguing. After with just over 2000 miles on the TC now, I say it's more comfortable for me (and my wife) than the Grand America was. I do miss the GA's electric windshield, but the TC's standard windshield is good for my height (6'1"). In our colder Southern California months (like now) I would like to have a bit more windshield height, but I suspect aftermarket items will soon become available to correct that.
I've also purchased the R18 Bren Tune which IMHO really opens up the R18's engine power. The downside is less fuel economy, but for me the multiple benefits of the Bren Tune out-weight that one negative.
One thing that still needs some work is the Navigation. BMW has recently released a new TFT update (version 012.008.40) that is supposed to fix remaining Bluetooth and connectivity issues, which I plan to schedule to have done after the new year. But map legibility is kind of another issue when you're not navigating a route - smaller roads are difficult to see but setting night mode helps some. Some folks may not like the phone-based navigation these new TFTs use, but aside from my mentioned gripes it works very well for me. The TomTom maps which the navigation uses are updated on a regular basis, and they don't take a lot of space on your phone because you determine what areas you need to download for your routing.
So... knowing what I know now, would I still pick the R18 Transcontinental over the K1600? This is a personal decision for folks, but for me the answer is a definite yes. The TC just fits me better, and I really like the retro styling it has. Everywhere I go, I'm always complemented on it. This bike handles a lot better than I thought it would, having a 35 degree lean angle that only scrapes the floorboards when it's really being pushed in the corners. Normal riding in the twisties is not an issue, even with a passenger. The bike's auto-adjusting of the rear suspension works very well, where with the Grand America I had to specifically set it for riding with luggage, a passenger, etc, and sometimes forgetting to set it when I needed to.