I don't even have a smartphone (so desktop viewing only), and I generally prefer vertical scroll style. My biggest reason is because the text/font tends to be bigger and much easier to read. As a reader, I am RUTHLESS about legibility. If a comic isn't easily legible, I will not follow it, no matter how amazing it may be.
But if that were to be taken out of the equation (after all, it's not like you can't make a traditional page format comic with a big font size -- or a vertical scroll comic with a small font...), I think I might have no preference? It would come down to how well the creator utilizes the format of their choice.
BTW, I wouldn't consider Countdown to Countdown a vertical scroll comic (this is arguable, so that doesn't mean your categorization is wrong). If you go to their website, it loads one panel at a time, like Ava's Demon. I think that's the way it's intended to be viewed for that comic. Sure, the panels are stacked vertically and have the appearance of a vertical comic, but I think I see some indications that it wasn't designed specifically for that kind of viewing.
My LEAST favorite is the wide page format, where you do the Z but it's a very wide Z. I find this to be physically uncomfortable to read on the computer, at least with the examples I've seen.
My own comic uses the vertical scroll format. Despite font size being a big factor for me as a reader, it wasn't why I chose that format for my own comic. I originally published it on Korean sites, where vertical scroll is the norm. It was a cater-to-your-demographic thing. I consumed lots of vertical Korean comics before committing to the format with my own project and found some amazing examples to love and learn from.
After a reboot, I stopped publishing it on Korean sites, but I kept the format because now I can't imagine this particular story any other way. =P Doesn't mean I'll never go back to the page format with future projects, though! Every project is different.