The Mac Mini i7 renders video faster (can rip and rencode a DVD/Blu-ray) faster than any of the stock iMacs even in the current gen. I actually use mine for video editing all the time and it works great! Probably the best Apple computer I've bought in the last 5 years in terms of speed & value. It's essentially an i7 Macbook Pro in a small low powered box.
So essentially it runs like a standard iMac or MacBook Pro just w/o an apple display, keyboard & mouse. Does that low power affect anything ?
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For your stated needs a Mac Mini will be perfect. Usually the case for an iMac is if you don't have or want to purchase a keyboard and mouse and separate monitor, or if you simply want an all-in-one device. (No denying how slick an iMac looks.)
Life span really depends on what you purchase from the start. I bought a weak spec'd Mini in 2010 and it really slugs along in 2014. That was my mistake. Had I bought a model with proper RAM back then (say 4 GB) then I'd be running Mavericks instead of being stuck on Snow Leopard due to my 1 GB internals.
I suggest you wait until the Mini is updated if you can. (Hopefully this news item turns out to be true.) I'll be getting a new Mini as soon as it's refreshed. At the very least it should be by October but hopefully before September now.
Thanks for the infio & yes I was gonna wait for a refresh. 1 more question if u don't mind. If I plan on running Plex, do I have to purchase the model w/ OS X server or can I do without ?