For street, definitely the 21. I am currently committed to using a 40 all month for this forum's Single in challenge, while at the same time there challenge for my local camera group is street photography... And it's frustrating the hell out of me, because with the 40's mild telephoto effect on aps-c most of my shots are more like candid portraits rather than 'authentic' street shots. The 40 makes it harder to include environmental context, unless you stand further away, which changes dof etc and takes away the feelings of immediacy and rapport with the subject.
The 21 is sharp enough to crop if you sometimes only need the area of the shot that is equivalent to the 40. The only reason I would buy the 40 is if you could get a 40XS dead cheap, like I did. If you don't want to pay the price of the 21, I would get the 35 f2.4.
Last edited by Bagga_Txips; 04-08-2015 at 10:44 PM.