In praise of primes - which next?

I agree that 21/40/70 is a more useful trio than 21/50/70. Not at all coincidentally, it's the trio of primes I use. While the 35mm is tempting, I chose the 40mm instead because I much prefer the construction quality and the aperture ring of the Limited lenses. I also agree that the 15mm Limited is an attractive lens; it will be my next purchase.

The 40mm is a lovely little lens which provides a photographic experience nicely complementing that of the 21mm. Think about using the 21 for walkabout street photography: since it's wide angle, you look for images of interesting things in interesting contexts. With the 40mm, slightly telephoto, you look for images of interesting things you can isolate from their contexts. The 50mm provides the same difference, of course, but it's a bit too long for my taste (and my street experience). And since the 40mm is a pancake, people tend to ignore it. Your mileage may vary, of course.