With the right tool, a cotterless crank is easy to service. Otherwise, you can't do it at all.

Similarly, with the right tool, a cottered crank is also easy to service. The difference is that you can service a cottered crank without the proper tool; unfortunately it is a *****. I don't recommend it.

For many many years, the best bikes came with beautifully made, heavily chromed cottered cranks. They are excellent cranks. Then, for a short time, only the best bikes came with cotterless cranks; but as cotterless cranks trickled down the product line, cottered cranks became the sign of a cheap bike. A cotterless crank became something desirable in its own right; the quality of cottered cranks fell sharply, and became something to be avoided.

Today all bikes come with cotterless cranks, so the debate is effectively over, as far as new bikes go.

I recently serviced the BB on my 1950 Norman Rapide. The cranks are Williams, nice but not top-of-the-line. While I had the left crank arm off, I put it on a scale: it weighed seven ounces. Then I weighed a cotterless crank arm, a Shimano 105, ca 1995, very beefy looking: also seven ounces. To be fair, the Williams arm is 165 mm, the Shimano one 172.5.

Almost 60 years old, the Williams chain ring is like new; the teeth have not worn down at all.

All things considered, I don't see that a cotterless crank is necessarily better than a cottered one.

That said, there certainly are situations where a cotterless crank is better than a cottered one. If you're looking at a mid 70's Raleigh, for example, the one with the cotterless crank is sure to be a better bike than the one with the cottered crank. But don't fool yourself: changing the crank does not make a Super Course into an International; and even with its original cottered crank, the Super Course was a fine bike.