In my experience, a 4 stroke engine will last several times as long as a 2 stroke engine, sometimes as much as 10 times longer. On the other hand, the top end on a 2 stroke, if you have the right one, can be quickly, easily, and cheaply rebuilt, while a 4 stroke is much harder, more time consuming, and anything but cheap.
To make a comparison, I have 2 scooters, both bought new. One is a Yamaha Vino 125 4 stroke. I have 14,000 miles on it, and have not had a single problem. Many people have put over 50,000 miles on these engines before having any problems.
My other scooter is a Genuine Stella, just bought new a couple of months ago. It is a 150cc 2 stroke, which failed on me at 400 miles. It was repaired under warranty, but according to all the scooter forums I belong to, 8,000-10,000 is the absolute max you can expect from a Stella top end. On the other hand, a new one costs $249, and you are back in business. A Yamaha motor cannot be rebuilt with new parts for what a scooter with 50,000+ miles is worth. You are looking at about $1200 worth of parts.
For a bicycle motor, considering the low cost, I am definitely going with a 4 stroke, as much for quietness as anything else.
One more thing. Not all 2 strokes are short lived. I managed to put 20,000 miles on a 50cc Tomos moped, and never even had the head off. It was still running great when I sold it. Build quality and quality of materials used seems to have a lot more to do with how long a 2 stroke lasts than a 4 stroke. My advice is whichever way you go, stay away from anything made in mainland China. 2 stroke or 4 stroke, it's all junk. Jerry.