The image retention is a total non-factor in every day use IMHO. Yes, I did Marco's test and there's some minimal retention that dissipates quickly and there's no way it's detectable in everyday use. I tested a couple more at the Apple store and they had it to varying degrees but I really don't think this is something that users will notice unless you're running something like that test (which is very atypical).
Performance I did notice, but only on a couple of fairly intensive games -- Infinity Blade III was definitely smoother on the Air, but not enough that I'd want to sacrifice the form factor of the Mini. Just buy what you want depending on your needs and it should be fine.