Wheelsize alone does not necessarily dictate ride quality. A combination of wheelsize and "wheelbase" make a bike either ride well or ride worst. The problem with the 16" option is that a lot of makers are emphasizing more on the fold rather than on ride quality because that's what people are looking for in a smaller sized wheels. People need to realize that you can't have the cake and eat it too when it comes to folder bikes; but sadly a lot of people want just 1 bike but a bike that does many things. If you are willing to compromise a bit and go 16", I would say the Brompton is the best option thus far in terms of compact folding and ride quality as it has a long wheelbase to begin with, just as long as a full sized touring bike!! And that helps contribute to great handling and good ride quality plus the Brompton comes with a rear suspension elastomer to help smooth out the ride.

Again. In terms of folding, a 20" folder bike isn't any BIGGER in terms of footprint than a 16" Brompton despite what many are trying to say here. They both fit in trains and buses and qualify as carry-ons and the Dahons and Terns will fit in a legal size 62"suitcase with room to spare. You can compress the fold on the Dahon a bit by doing the "N" fold so the 2 magnets don't touch. Tern does N fold, but I've been doing the N fold on my Speed Uno and Mu SL for the longest time and have no problems.

A 15 mile commute is not a problem as long as the bike fits you or any bike really. Laura Crawford and Russ Roca (of PathLessPedaled) tour with their Bromptons and the last time I've talked to them which was this year; they've been riding far more than 15 miles and with heavy backpacks and front Brompton bags doing cycle camping no problem. I couldn't feel at all comfortable with a Brompton for as long as I tried cause it's designed to be super uber upright riding and I prefer less upright. So when you're looking to buy a folding bike, ask yourself this question. Why do you need a 16"? So you can store it in your closet? So you can ride the tube/skytrain in the peak hours where people are packed like sardines? Or your transit service charges increase for 2 to 3 zones fare in your city so you need to evade and save on less zones?
On my daily commutes, the only times I see people riding a Brompton is when they are doing one or all of the above. For example, a Brompton rider I see everyday commutes from Richmond to Vancouver to catch the Vancouver train. If the person takes the train in Richmond, that would be a 2 zone but if the rider simply rides 10km to the Vancouver station which is just across the water, it would be just one zone. That's a saving of $29/month, roughly $350/year which makes the Brompton ownership easier to swallow as it will take roughly 3 years to pay it off plus the savings you can make by reducing the reliance on your car thus lightening the repair loads.
As you can see here, there are a lot of factors that determine why people choose 16" as opposed to 20". If that same rider chooses to ride to work in downtown Vancouver from Richmond every day like I do, then I will forego the 16" and go with 20" as my Dahon Mu SL have way superior V-brakes than the weak weak Brompton brakes especially going down a long steep 18% hill in the rain which is part of my commute plus the Thudbuster ST and Big Apple tires provide the smoothest ride you could ever get with a 20" folder.