Both are excellent and you can't go wrong either way (I presume you're referring to the 25mm f/2 model and not the f/2.8 model). Comparing the sharpness numbers over at Photozone.de, the 25mm is significantly sharper in the middle of the frame, but is less sharp than the 21mm on the edges and corners. I think that may be why the 21mm is so well-regarded: it sharp enough in the center, but keeps on staying sharp across the frame. And that's not to knock the 25mm; it is still impressively sharp across the frame as well - just not as consisently as the 21mm.
I never had any complaints shooting either lens - I love them both equally.
Since you mention architecture, have you considered neither Zeiss and just getting a Canon TS-E 24mm II? It is much more versatile than the standard lenses, like the 21mm and 25mm, yet is sharper still and incredibly well-corrected for distortion, flare, CA, etc. For me the Zeiss 21mm or 25mm are very-nice-to-have lenses, but the TS-Es are absolute must-haves (especially for architecture).