There is also price. I bought my 2000-vintage Sun EZ-1 about five years ago, used for $500, could not have afforded more -- and it has been marvelous, I plan on riding it or one like it until I can't ride anymore. But after a couple years I had the rear 7-speed gears with an 8-speed internal hub and had the front derailleur removed, almost doubled the overall cost, but raised the reliability of the whole 10x. I also put a 2.3x16 scooter tire and tube on the front, for higher pressure and air retention, probably at a cost of some weight. But I can just grab it and ride if I pump tires once or twice per week.