Okay, first, I personally never use qwerty. I set up one slot for it on the device I take on trips in case someone else may want to try it. If I had more slots, I'd probably have both layouts for all of them, just in case, but not necessary.
If I was going to switch from qwerty today, I'd lean towards Colemak. I'm not sure it is better than dvorak. I've seen places that say it is, but that may be from people who use it and not really a scientific comparison. But, in any case, I think the difference is minimal so certainly not worth switching from dvorak to colemak.
My keycaps are qwerty, like all testers. Not a big deal since I've always had to use a qwerty keycap set. The symbols for dvorak and qwerty on the TB are mostly in the same places (but not all) so that isn't bad.
I worked hard decades ago, with a computer typing program, to learn qwerty touch typing. I forget my numbers - that forgetfulness is fairly recent, since using the TB, I seem to have filled my head with so many changes that it's pushed that decades old info out!
But back then, if I practiced on the program a lot, I know I got over 40 wpm. But if I stopped testing, even though I was still typing, my ordinary tasks didn't involve as much typing as I do now. So my speed would slip into the lower 30's, I think, until I did more formal practicing.
Eventually I decided to try dvorak because I wasn't satisfied with my speed (I don't recall anything about accuracy). Certainly slow at first, but it didn't take long (I'm guessing 2-3 weeks) for me to be as fast on dvorak as I had been on qwerty - but I kept getting faster. Certainly into the 60s. And when I didn't practice for awhile and checked my speed, I'd drop down, but it was into the 40s - so my slow speed on dvorak was as good as my fast speeds on qwerty.
But like I said, doing the same thing today, I'd lean towards colemak. For one thing, I believe fewer keys are changed so that alone should make it a little easier to adjust to.
Because some keycaps won't be available when the TB is released, WT has said they'll send out emails where you can see what is available and maybe an estimate of how long a wait it will be for other layouts so you can decide what you want to do. Personally, after getting to use the TB, I ordered a second one so I'll actually have one for dvorak and one for qwerty as far as keycaps go. I'll normally keep the dvorak one at home where most of my typing is done but put the qwerty one in my bag so I don't have to worry about forgetting it. And I like having a backup anyway so I don't have to go back to standard keyboards if something breaks. But both will be set up to use dvorak. Only the keycaps will be different.