I have not used any of those in a long time, but this is what I remember...maybe some architects can correct me if I am wrong.
FormZ: one of the best modeling(technical)/drafting software you can get for the mac. Pretty much lets you draw/build anything. Fantastic from 'rapid prototyping' to detailed final drawings/renderings. Definitely the way to go if you are more about representation than project planning ~$2000
Vectorworks: great 'integrated' package. 3D tools do not offer the same level of control as FormZ, but you get extra database/spreadsheet/workgroup stuff that can help with the workflow of an architectural firm. Very affordable (starting at ~$900)
ArchiCAD: 'The classic'. Pretty much a standard, but it might have lost some market share to Vectorworks (just an impression). Can be extremely powerful if you know what you are doing, but it also requires a bit more learning and planning. Drawing is not quite as 'immediate' as in other apps, but objects (walls, windows etc.) are pretty much parametric so you can easily change them without having to redraw everything. You also get database links that let you calculate material costs automatically and so on...
Great if you are working on 'real projects', but it is also one of the more expensive options ~$4000 (unless you are a student).
Hope this helps. Look out for student/demo versions first...