A point to bear in mind is that there are some (imho very) minor issues with the HD3000 movie playback which are fixed in HD4000. Something to do with not getting the 23.976p framerate right and stuttering and lip-synch issues. Can't say I have ever noticed myself. But if it's important to you, probably worth reading up about this before buying a 2011 Mini with only HD3000 graphics. (Or get one with a graphics card in it.)
Probably nothing to worry about, but thought I would mention it since movie playback is your intended use-case.