I think they'll both suit your needs - they're plenty strong to do basic photo and video editing. I mean, my powerbook is just 2 years old and was as powerfull as laptops go - a pro-machine so to say. And today both the iMac and the mini has much faster processors than my pb. If you really want to you can upgrade the ram, you'll really feel the speed increase, but you should also be fine with the 512 they come with.
On a side note, iMovie is really the perfect amateur video editer imo. Extremely easy and intuitive to learn and use.
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