Well, I agree that there is a huge subjectivity factor, but some things can be compared on a reasonably level field. I agree that you are getting more hardware with an iPad 3, but the usability may not be too different considering that it has to drive 4 times the pixels of iPad mini. I do think mini's 512 MB is on a rather stingy side, but so is 1 GB on iPad 3. Again, for 4 times the pixels. So which one will age better is arguable.
I am never a one to advocate downgrading for convenience or aesthetics, but there are real and practical benefits from lighter, better-designed form factor. Web browsing, for instance: Most sites nowadays have mobile-oriented versions which work rather well on 7~8" screens. Full blown desktop sites fit in 10" screens, but in order to read you will need to zoom anyway.
So that leaves one concrete advantage of iPad 3 which is HD playback. You can watch 720p/1080p material without any conversion or downgrade on iPad 3, while that's not possible on iPad mini. If this matters a lot, then perhaps iPad 3 is a wiser choice. But at that point, shouldn't you go with iPad 4? It's been discussed and acknowledged that iPad 3's GPU is not enough for 2000x1500 under many scenarios.