Mac Pro all the way
The i7 iMac is a *very* solid performer, but if you can front the budget for a Mac Pro I wouldn't hesitate to go that route. The MP has a longer useable lifespan (mine is an early 2008 and it's still faster than just about every machine Apple currently offers), a higher RAM ceiling (32 vs 16), plus 4 internal hard drives (or more with adapters).
You could plug in 4 drives, use 1 for the Boot/Apps, and then RAID stripe the 2 bays for a super fast scratch/data drive for her photos and videos, and put a nice new 3TB drive in the 4th bay for Time Machine/Backups. SO much faster than external drives and you don't have to suffer through that annoying external drive windup that always takes a few seconds.
Blah blah blah I could go on but bottom line, if you can pay for the MP you won't regret it. At all. At 3 years old mine is showing very little age vs other computers. I totally expect to use it for 3+ more. The only place it's getting shallow is the GPU, but that's easy to fix with a new one.