[Nicolas06]

You should first see what focals you like. The ones you make great photos with, the ones you can let on the camera for a while and be satisfied on what it does!

For me, it is the 21, more so than 15mm or 35mm for example. I can keep the 21 for many shoots and be very satisfied of the result. You can both take photos of landscapes, some street photos, make very nice closup of flowers or small objects... All in one tiny lens package. This make it wonderfull for wandering arround !

Sometime 21 is too narrow, for photos of many buildings as an example or some few dramatic scenery that really benefit of wider angle of view. But in theses situation, I have the feeling that 15mm would be too narrow anyway and then I think more of something arround 10mm... In Florence, Italy for the duomo I did not take the entire building and I'am 100% sure the 15mm would have not done the job neither. What I enjoyed on the contrary what using my 50-135 to take some shoots of architectural details and that was really nice too. I was also very satisfyed to go on the top and make many tele shoots of far away monuments in the city from this upper situation. I go shoots that even the best UWA lenses would not have achieved this way!

So you see, it depend on how you fell. I'am not better I juding the right focal lens than you, I may have not the same test. I could also change my mind.

Anyway, not to say you should go for 21 instead of 15, or that you should not use your 30mm or whatever.

No, what I mean, like a few others already said, you should really look for the focals you like, that'd you be happy to use a lot and try to see what lenses are available for that. Don't try to build a continuous set. Nobody care. Anyway if you just want to crop it is fairly possible: use the 15 when you need 15. Use the 15 when you need 21-24mm and crop in post processing. You'll be able to fine tune you composition in post production and get rid of the corners/border where the lens could be less sharp! If you have a 24MP K3 without filter, cropping to 12MP will give you more than you need, and corner to corner sharp.

If you didn't take the 15 yet, I think you canno't be sure you need another lens between the 2. My advice would be to get the 15 if you want it, but used, try it and see if you like or not. You can alway sell it after to get a user 21 or something without really loosing money on it.