Pebble, early this year I looked into moving to 6x12, did much the same investigating that you've started. A couple of things leapt out of users' comments on the 'net.

I found many complaints about Cambo/Calumet roll holders. I'm sure there are happy users, but there are also unhappy ones.

I found nothing but praise for Horseman roll holders.

I found some complaints about Sinar roll holders with adjustable gates. They don't always adjust properly, and sometimes the gate opening isn't perfectly rectangular. And complaints about Sinar backs' weight are fairly common. I don't understand them, my 6x12 Panorama weighs 779 g. Not that much, really.

Very few complaints about DaYi/Shen Hao (they are the same) 6x12 holders. The worst thing about them is that they don't have auto-stop. Wind until the right number appears in the red window.

I have a weakness for slip-in roll holders, eventually bought a used Sinar Panoramic. It failed shortly after arrival. There's a lever for switching it from "load" to "advance." The lever broke, I got another and advice about how to do the replacement from HasselbadBron. Not long after, someone asked for help with the same problem on the French LF forum. He and I discussed it, he approached Sinar and was told that the part is no longer available. I think he got one from HasselbladBron.

The Sinar holder's film chamber interferes with the "ears" at the right side of a Graflok back's focusing panel. I can force mine all the way into a 4x5 Cambo international back, but its a little precarious, so I use the back's Graflok sliders to make sure the roll holder won't wiggle out.

If I had it to do over again, I'd get a Horseman roll holder. Don't misunderstand, since I replaced that lever my Sinar Panorama's been fine, but its advantages over the Horseman aren't as great as I'd thought and it is quite old and no longer supported. If you must have a Sinar roll holder, check with Sinar that the version you settle on is still supported. Or get one of the Chinese (relative) cheapies and pay attention when you advance the film.