I own an NEC LCD2490WUXi, and have had very little experience with the Apple 23" LCD. As for any concerns about apparent "sharpness," all you must do is sit back a few more inches from the screen, and when the 24" occupies the same area in your vision as the 23" would if you were sitting a bit closer, then there should be no noticeable difference in sharpness.
Since I don't own both, all I can do is tell you why I bought the NEC over the Apple. The NEC can be calibrated with its Spectraview software and an Eye One Display2 (or possibly other) colorimeter. This calibration is done within the display at a higher bit depth, so once calibrated, the NEC should be able to use all of its 256 levels of brightness per channel, while the ACD will have lost some due to rounding after the calibration curve is applied in the graphics card. Smoother gradients are the result; all I can say is that my NEC is noticeably better than the Samsung it replaced, which lacked internal calibration as the ACD does.
The NEC also has multiple inputs and a very extensive and useful menu system - two things that His Steveness would never allow to exist on an Apple product. This flexibility lets me have both a MacBook Pro and a PC desktop connected to the monitor at the same time, with a third input available if I need it later.