Generally, the greater zoom range compromises optical quality. The tradeoff being versatility of course. The link I gave to photozone has lots of lens reviews including centre/edge sharpness at various apertures and focal lengths.
However, the 12-24 is an ultrawide angle and a problably very challenging to design to start with, so that in itself will compromise the sharpness. There is a review for 2 12-24 lenses here:
http://www.photozone.de/8Reviews/len...4556/index.htm
http://www.photozone.de/8Reviews/len...24_4/index.htm
The Tokina 12-24 looks excellent.
Terry