I have both, and unless I need the extra speed, I use the 21f4.5. If you are shooting precise architecture - go for the f4.5 as it has virtually no distorsion. It is also compact enough to be carried as a lens in a camera bag.
The 21f2.8 is for when you need that extra speed (low light, dim interiors). It has more "curve" at the edges - but no more so than most other premium wides. It is a bit bulky compared to the 21f4.5 - but not "huge" by any means (the really big one is the new Summilux 21f1.4 from Leica - but you can buy 6 of the 21f2.8 for what that one costs!).
If you are going to use the 21 as a prime lens, go for either the f4.5 or the f2.8, If you are looking for an occasional wide lens, look at the Voigtlander Color-Skopar 21mm f4.0 in M-mount. Very respectable performance - and actually less distorsion than the 21f2.8 Zeiss (or the Leica 21/2.8's). Not as bitingly sharp as the 21f4.5 Biogon - but in most cases you will not see the difference as any of these lenses will out-perform the film you are using or the sensor in your digital Rf.
Go to Flickr and tag the various lenses and check what the end results look like. resolution is impossible to judge on the screen, but distortion, edge fall off etc can be compared.
