Not a big deal about the questions. The search function on this site is beyond worthless. Any time I want to search for something I use google, and just type "site:Buellxb.com" in with what I'm searching, so it would be something like "site:buellxb.com tail swap". I'm at work otherwise I'd do a little looking into it for you.
I'm not sure what would be easier. I think I see more firebolt tails on lightnings than I do lightning front ends on firebolts. The difficult part is figuring out what to do with the ECM(computer) and the fuses. On an xb9r those things are up front by the speedometer, and on the xbs they're jammed under the seat. If you switch to a lightning front end you would have to try to fit those things under the seat, but the problem is that the plugs are at the front of the wiring harness so you would probably have to get an xb9s harness and put it on the xb9r, unless you're good at electrical work and want to cut up and solder it back together.
I would venture to say it's easier to put the firebolt tail on the lightning. You would have to find a new place for the ECM and fuses since the lightning tail has designated places for them, and the firebolt tail doesn't. I would assume you could just stick them in the space under the passenger seat, not sure how big the space is though. You would also have to get a longer line to the brake reseviour and a longer cable for the seat latch since they're in different spots as well.
Hope it helps, and I'm sure someone else with more experience will be around eventually. Also, if you know that's definitely the setup you want, you can probably find someone on here who will trade. People mix and match their xbrs and xbss on here all the time. I see lightnings with firebolt tails, and firebolts with lightning tails, so whichever you find, you can probably find someone who's willing to just swap parts with you to make it work, especially if it's winter and bikes are parked anyway.