My preference, personally, is based on the sound I want. The larger the speaker, and the more speakers you have, the deeper the response is. That can go to the point where you have a really really really deep bass cabinet that can sound somewhat muddy (which is sometimes desirable, if you like dub and reggae).
Personally I think a 4x10 gives the most stable spectrum of clarity and depth of bass response without muddy or twangy. Often, you'll be going though a PA system as well, so you shouldn't have to worry too much about volume when it comes to cabinets.
Right now, I'm using a Markbass 2x10 for smaller gigs (which is one of the best cabinets I've ever used by far!) and an SWR 4x10 for larger and louder shows. I haven't come close to needing both cabinets on the same show yet, thanks to PA systems and decent Front of House engineers.
Larger speakers are also going to have a more loose and slower response, which can also be desirable. 10's sound very tight and punchy.