I switched from a VTX18C to the FJR - never checked the C14 (technically more appealing than the FJR, but the exhaust aesthetics killed it for me, not to mention the inconvenient and expensive maintenance).
If C14 is out, BMW ruled out for price, and ST1300 for its ho-humness, then you are almost left with nothing but the FJR. Yes, there are others like Ducati and Triumph, but normally these are not mainstream bikes that people consider.
IMO, there is no comparison between a cruiser, like the VTX18, and a sport touring bike. However, since you asked, I will give it a shot too.
Handlebar vibes/buzz
VTX - Non-existent. The whole bike has nice "thumping" cruiser vibes as intended, but no irritating, numbing, "get-used-to-it-sissy" vibes. On long rides, relaxing your right hand never comes to mind.
FJR - The throttle grip can be nasty depending on whether you "get-used-to-it" or not. Some kind of cruise control / throttle lock is highly desirable to relax your right hand after 20 minutes of riding.
Power
VTX - The 1800 is known for its massive torque, but slow down the bike in a severe climbing twisty and you have to shift to lower gear(s). Try to go into triple digits, and you have to wind it up like an aircraft on a runaway before takeoff. But, in fairness, a cruiser is not designed for those kinds of speeds.
FJR - Much smaller engine (1300), but the torque is amazing. You can be slower than 30mph and still be in top gear and smoothly take off to beyond 100mph. Between 90 and 120mph the bike seems very much at home if you wanted to cruise in that range for hours. Bike cruises very well at lower speeds too, but normally the rider loses patience with so much power available at the tap.
Fuel Consumption
VTX - Nasty, especially if you are riding faster than a cruiser's intended cruising speed. As a consequence, on long desolate roads you many-a-times go into a panic mode as the gas tank runs low very fast. The ride has to be planned carefully for mileage, and frequent gas stops are needed.
FJR - Fantastic; mentally very relaxing, and gas stops are not a priority anymore. Automatic fuel reserve trip odometer is an added boon.
Comfort
VTX - I used to have lower back problems and the VTX suspension would jar my spine going over potholes and the like. I am not sure if it was unique to my bike's suspension, but reading forums it was clear that the most important farkle for most owners was suspension upgrades. This was the main factor for me to switch to a sport touring bike as it made me lean forward, thereby mitigating the back issue. Though the VTX is heavier than the FJR, its low COG and seat position never makes you nervous about tipping the bike over.
FJR - Suspension is smooth as silk. Potholes don't bother me at all anymore. Bike is top heavy, and prone to tipping over if not careful. High seat would make one tip-toe if your inseam is less than 30".
Seat upgrades were necessary for me on both bikes (due to my flat butt anatomy). The FJR stock seat is very painful for me, and a cheaper upgrade (Spencers) helped a little, but not much (btw, Spencers is a fantastic vendor to deal with).
Storage
VTX - The "C" trim didn't have any. I used to carry a duffel bag on my pillion seat.
FJR - Great, coming from VTX's zero storage. The hard cases are really big and deep, and yet very sleek. The bike's aesthetics is not spoiled at all, unlike C14 and Ducati, IMO. Also, it is very easy to remove them for overnight stays. I always ride with them on, and speed is never an issue. From a visibility-for-a-cager viewpoint, I would think a wider rear end is better than a sleeker one w/o bags.
Heat
I felt it more on the VTX (though completely open, maybe that's why), than I feel it on the FJR.
Color
VTX - Quite a few choices.
FJR - Black for the latest ones. I was skeptical as many are, but had no choice. But, having owned it now, I would not settle for any other color. It makes the bike look even sharper. My friends, on other brand motorcycles, also have the same reaction when they see the bike in person compared to seeing it on a website.
Cost of Ownership
VTX - OEM spare parts are quite inexpensive. Needs pain-in-the-*** valve adjustment every 4K miles. Lower insurance cost. Higher gas cost. Wide negotiating latitude with dealers for new bikes.
FJR - Everything is opposite of VTX.
Forum
VTX - CaliforniaVTXRiders forum is excellent. Technically some highly qualified members, and never experienced getting flamed for not searching a topic and posting a question. Several would repeat the answers, or provide links to previously answered topics.
FJR - Great members too (look at the response for Tyler), and technically very sound. However, sometimes patience runs thin, god-forbid someone has not researched a topic well or missed it in a search and posts a question. (Age can't be the excuse since VTX riders fall in the same group.
)
Summarizing
I would never ride a cruiser again once having tasted a sport-touring. I had rented a FJR before I bought one, and after my trial I felt crummy riding the VTX. But, the VTX looked gorgeous too as does the FJR!
There are minor issues like older technology, uncomfortable seat, and top-heaviness of the FJR to deal with, but the handlebar buzz could be the deal breaker for me if a competitor came out with a better bike that is similarly priced. That is the only snag in long-distance enjoyment.
