My feeling is that for your needs (photo and video editing), you would do better with the 27". This is assuming you're happy with the concept of the mostly non-upgradable iMac. I have a few reasons:
1. The resolution of the 27" may be too high for native 1080p, but in a video editor, the pane that shows the video will be larger, and show you more detail of the source.
2. The better graphics card will help in editing with most newer editors.
3. Your needs can be RAM hungry, and the 27" can be upgraded yourself, to 32GB, double the 21" max. As Apple solder the memory on the 21", you need to pay the Apple tax for 16 GB, and have to do so when buying the computer.
4. The 27" uses 3.5" 7200rpm HDs, instead of 2.5" 5400rpm ones - a big difference in speed. The 3TB upgrade (only 27") may help in keeping all your files internally as well. This can be combatted at least partially by getting the fusion or high speed external storage, but all are extra cost and/or clutter.
5. I have heard that the speakers on the 27" are much better than the 21", so you may be able to rely on them and avoid using extra space with externals.
All in all, if you have need for something relatively high end, the 27" becomes better value.
Hope this helps,
David