Jake,
My wife and I broke into triking on a GL1500 with a CSC trike kit that had a solid rear axle. We loved it, but it was a rough ride across gutters and rough spots. Just the nature of the beast. We made it just fine and loved the machine. When we upgraded to our present 1800, after considerable research and considering the relative closeness of a kit installer we already knew, and with recommendations of his work from others, we settled on a RoadSmith trike kit with an independent rear suspension.
We have absolutely been pleased across the board. Both my wife and I will tell you that this machine is the most comfortable trike we've ever ridden, and we rode several different ones prior to making our final decision. The longer wheelbase of the unit no doubt contributes somewhat to the comfort, but this rig is smooth, and we like it. It not only rides very well, but it handles very well. We have the 5.5 degree rake kit installed on the front end. The machine runs straight down the road till I get ready to change direction, and it does not require lots of effort to change direction. It handles well in the curves (understand I am not and never have been a hard charger through the curves, plus there ain't very many of 'em in our local area, but we ride over to the mountains quite often and there are plenty there!).
It would be very, very difficult for me or my wife to go back to the solid rear axle trike. We are sold on the IRS, and we are particularly sold on the design and engineering of the RoadSmith kit. There are other good ones, but we have not experienced some of the difficulties that other owners of other brands have had to deal with. The experience and quality of service provided by your trike kit installer is likely more important than the brand you buy, but you will be hard pressed to find a better machine than a Roadsmith conversion. That's my seriously considered opinion. I'm as skeptical and hard to please as most anybody, and I have yet to have my first serious criticism of this machine. It may come yet, but so far, so GOOD!!!!