Omega for what it's worth I think that you're leaning toward the 1300R for the right reasons.
A bigger guy needs a beefier horse. And feels better on one too.
We may never know, but I suspect that the surprisingly high weight of the 12 is overly-concentrated in the new frame for safety's sake, whereas the weight of the Busa has been more evenly distributed with more robust construction almost everywhere.
Main frames aside, the Busa seems substantially beefier in the sub-frame, transmission, triple clamps and wheels.
I don't know about the forks and shock, but those with specs for both bikes at their fingertips may find some differences there, and that would be crucial.
Also, many of the non-structural sub-assemblies seem beefier on the Busa. Wiring, rearsets, linkages, instrumentation, bars etc.
Psychologically, the 12 has a petiteness of presence to it that makes the XX and the Busa seem big in comparison. This factor, which is more a function of design than actual bulk, may be irking you a little...especially given that you've owned at least one Harley.
Leaving the XX behind makes sense for you. Even though it may have the beef you need, it just does not have the performance futures built into it, nor the commitment from the aftermarket, that the Busa and the 12 have.
If I were your size, and if I modified and rode my bike the way you say you do, I'd be choosing the 1300r.
And screw $500 this way or that. Being comfortable with your most important material possession is the main consideration.