Hubs come in many varieties. I will focus on hubs that come on folding bicycles. 

Axle 74mm vs 100mm: Some companies use a 74mm front axle on their hubs. The narrow axle saves some space ( 26mm to be exact ). 100mm axles are standard in the bike industry, 74mm axles will be hard to source if necessary. I would stick with 100mm hubs for practical reasons.

Disc vs. Non-disc: Disc brakes are not ideal for travel, since rotors can bend. So I normally ride with rime brakes. However, I prefer the option of going with disc brakes. Hence disc hubs are superior to standard hubs. All of our 20" wheel models use disc hubs.

Seals: The quality of the seals defines the quality of the hub. Weak seals allow dirt and moisture to enter the bearings space. This will degrade the hub quickly. Overall, get hubs with nice seals.

Bearings: Most hubs use cup and cone bearings. High end hubs use sealed cartridge bearings. Get the sealed bearings if you can, they are super smooth.

High flanges vs low flanges: High flanges allow for the use of shorter spokes on small wheel bikes, this means the wheel will be stronger, hence less risk for breaking spokes. Low flange hubs are lighter and have a classical style. Overall I prefer taller flanges to make stronger wheels.