Also, most factory fit alloy options are exactly the same size as the steels they're replacing these days.
On the other hand, if you do specify a bigger alloy, you pay for it in tyre costs.
Some cars just don't look right on steels (the 156, for example), whereas others (the Passat) look OK on either. There's even cars out there that look much better on steels (MkII Golf GTI).