I commuted for 4 years on a 20" folder with a Nexus 8 in it. Over the last year or so, I've switched to a Dahon Curve "D8" with Sturmey Archer X-RF8 in it with 16" (305) wheels. My backup bike is a Cirus Smart (think Dahon Dove) with 14" wheels, so I'm coo-coo for small wheels.
My thoughts bikes with 16" wheels:
- the ride is a bit rougher with 16" wheels, but this can be partially compensated for with fatter tires such a Schwalbe Big Apples
- they feel more responsive or twitchy... some say unstable, but I like the responsiveness, especially for urban commuting
- Schwalbe Kojak slicks aren't available in 305 size, but you can get Primo Comets. Note that there is a Kojak for the 349 wheels (Brompton)
- studded winter tires aren't available in 305 size, but you you get them in the 349 wheels (Brompton)
- bikes with 16" wheels can have a shorter cockpit that bikes with 20" wheels. I'm 5' 7" and the Curve is just right for me after lowering the handlepost. Bromptons do have a longer cockpit
- often have narrow rear spacing, which means fewer choices for internally geared hubs.
- the smaller fold means you might be able to pack it in a standard airline case without disassembly
Here is my experience taking the Dahon Curve to Europe in a suitcase
Dahon Curve D3 in airline legal suitcase, North American to Europe
Note that the Downtube minis with belt drives are the Nexus 8 and the Alfine 11 models. The Sturmey Archer 8 has a chain.
Let me know if you have any questions.