This decision is doing my head in now. I like the size/portability of the 11" but am concerned that the extra power of the 13" would be better, not to mention the battery life and disk space.
At the moment, its a show-down between a new 11" ultimate and a refurb 13". I want the option to run a couple of demanding Windows programs but the main use would be surfing, email, possibly app development (tinkering at least) and dealing with photos - I've started shooting in RAW but am not doing much editing.. yet.
I went into the local Apple store earlier and was annoyed to find all their machines were the same - all base models in each size - so couldn't check the relative performance.