-> would you want a Touch Screen or No Touch Screen ?
Yes. But very likely only for the initial setup.
I have already started a thread with my musings about a laptop based setup
here, but in a nutshell, my hypothetical setup would likely include a Macbook Air, which I would only boot and then hide pretty much instantly. After that, I'd likely use an iPad as a Mainstage remote controller, but most likely I'd only call up the program and perhaps do some rough basic setup.
For all further finetunings, I'd add a MIDI rotary box.
-> reasons-thoughts for / against ?
I already cobbled together a setup (HX Stomp nested in a GT-1000 loop) allowing me to remote control all relevant parameters through a tablet running TouchOSC. And while I was extremely happy how it turned out technically, it wasn't great to deal with as soon as I wanted to make quick adjustments during the 2-3 shows that I used that setup for. I'm talking about the kind of adjustments you might want to do while you hold a note/chord (regardless whether it's corrective or creative). You're then standing there with the left holding a chord and with the pick in your right, trying to adjust things on a kinda crowded touchscreen with that very right hand - well, I found that to be a *very* unpleasant experience (and I even practised using that stuff at home). The lack of any haptic feedback is just plain bad IMO.
I then briefly tried the same setup with a Beeeh-riiiing-eeer BCR2000 (which I don't want to use as it's *way* too bulky) and it's been like night and day, IOW pretty much excellent.
So, in a nutshell: Any setup I could imagine would include a rotary controller of some sorts.