Need help with choosing an iMac!

This will be my switch to iMac from a long run of PCs and I want to get it right...

Now that the date has been finally released I have to make my decision and I need help. First off, the 27" is too big for the space that we would put it in. As nice as it is, just too big. My tech loving nerd side would go full blown 21.5" with the fusion drive but the more I have thought about it (Apple gave me plenty of thinking time) the more I wonder if I need it or if it is worth it.

What I plan to do with it:

- minor photo edidting

- minor viseo editing

- air play mirror the screen to my apple TV for watching online content. (this is a big one)

- your usual web, word and media storage.

- no gaming

- Going to be a media hub for my Apple TV

What I need to know is:

Is it worth the $500 (rough estimate) extra to go up from the base 21.5" and then get the fusion? For what I plan on doing with it can the base model provide me good performance and longevity.

Thanks for any input.