If you want with you that eliminates the iMac.
If you are doing serious editing I think that elminates the iPad of any size.
So starting looking at the MacBook Pro.
For the uses you've mentioned, I'd probably want the iMac/iPad combo. Video editing can require a fair amount of processor power, and the iMacs are far more powerful than their portable counterparts.
But...as Allan has pointed out, portability is something the iMacs don't do very well.
If you're only going to use the iPad for showing photos and doing very simple edits, then it would work fine. If you've got iCloud, you'll have Photostream going to keep the photos you put on either device available on the other.
From what you've described, I'd go this route. (But...if you said, for example, "I want to go on photoshoots and use my iPad to collect all of the photos I've taken then go back to my office and put them on my iMac..." I might suggest to you that your iPad will quickly run out of available disk space, and that a MacBook Pro would be a better fit.