Others have far more experience since I've only recently come into folding bikes, but I've hauled my Brompton to a couple of cities in Ontario and over to Atlantic Canada, as well as to work when I can. I have to say that the Brompton's compact fold is very nice. I test rode a Tern (nice ride), but the fold doesn't compare to the Brompton's in terms of actual compactness and "neatness". If compact and as-low-as-possible-hassle travel is even only a partial consideration, heavily factor in how well the bike folds up.
ben's advice is spot on: "I'm really starting to think that all this frame material / wheel size / saddle type & position / IGH vs derailer / etc. stuff is only relevant when you get to the last 10 % of the cycling experience. As long as the bike is comfy, safe, doesn't break down every 15 miles, and the rider likes it you're 90% there."
I think the corollary is also true. When you're not biking and you need to haul it around and even if that non-biking portion is only 10%... if the bike's folded configuration is a pain-in-the-saddle for you to deal with (everybody's different), you'll hate it or regret it. The analogy I use is traveling with only a carry-on or the "one bag" philosophy. When you travel, the actual travel is probably only 10% to 20% of your total time, but if you have the wrong bag, packed too much, or packed the wrong thing, you'll dread the actual travel disproportionately, regardless of how good the rest of the trip is.
I don't have any experience with 20" wheels, but I can see how in some cases having 20" wheels could give me a better ride or at least more options. If you're mostly confined to the city, I suspect 16" wheels would serve you 90% of the time.