To answer the original question of how I decided which keyboard is my favorite (so far): trial and error.
Starting out, I knew I wanted something sturdy, keys that feel like there's good tactile and some auditory feedback. I wanted a "classic" aesthetic, like something you could see on a computer from 15, 20, 30 years ago or more. Beyond that, I was pretty uninformed except for what I was reading on forums like this.
I bought a cherry brown-based board (KUL) and futzed around with buying keycaps to get it looking cool. I liked it! I could probably have just stopped there and used that board forever.
But I was also reading a lot on this forum and others about Topre boards, and the HHKB in particular. At the very beginning, before I'd bought the KUL, I'd been attracted to the aesthetic and form-factor of the HHKB, but couldn't (yet) justify blowing that much money on a keyboard. Now I decided I'd take the plunge and order one. I liked the feel of the HHKB even better -- particularly the way the switch resistance collapses and falls gently down to the bottom. And of course the pleasing sound, and the compact layout of the board itself. After using the HHKB for a bit, I don't enjoy the feel and sound of the KUL nearly as much as I did before. Cherry switches that felt solidly mechanical now felt and sounded loose and rattly and scratchy, and just nowhere near as smooth as the Topre switches.
I'm sure there's some board that would open me up to some other experience, but I've already blown hundreds of dollars on keyboards. I'll be happy to just bang on this HHKB for however many years it lasts.