I know that you get a more shallow DOF and better high ISO performance with a full frame over a crop, but with good lenses on either is there really much of an IQ difference all other things being equal.
I recently added a 6d to my 60d, so I would say that next to intensive research I know both sides pretty well by now, so here's my general take: It is true that it is very hard to tell a unprocessed shot from a crop at low iso from a ff (at the same dof of course), and iso noise doesn't matter if you are shooting for key chain holder print size. *But* there are hidden aspects:
* ff has more leverage for postprocessing
, this is what matters the most to me. If you try to raise one color channel on a gradient (say lessen or intensify blue on a partly cloudy sky) the crop shot will immediately fall apart while the bigger ff pixels allow for more postprocessing creativity or fixing. Also the ff has less problems on the red color channel on plain surfaces, a known issue with Canon sensors, try shooting a red mushroom if you don't know what I'm talking about.
* The recent ff models like 6d have virtually no banding
(well, I've never seen it), quite unlike the 60d, not to speak of the older 7d - I don't know if this has gotten any better with the 70d. Banding after raising shadows is what kills your shot while you can reduce the film-like iso noise on the recent ff models a lot esp. with newer algorithms like DxO PRIME. In reality, I find the ff has a 2-3 stop advantage over crop because with crop iso800 is the highest "safe" setting while with the 6d it's iso6400, and truth to be told I'd still prefer the latter if the dr is low.
* Crop is equal to ff with good lenses
, but you really have look out for them or use ef-s - my midrange 17-40L and 70-300L are much sharper wide open on ff, though the tele zoom has a radial bokeh on the outside that you can like (or not) that is cut away by crop.
* Noise: I'd like to advise not to underestimate the freedom this gives you, it's not just for very large prints nobody will really do but with crop you constantly have to trade shutter speed vs. iso noise
as crop @iso800 is already problematic. Unless the dr is very high with ff you can just use a higher shutter which results in much more keepers, less headaches about tech but more thought about composition.
* If you use Magic Lantern: with ff you can boost your dynamic range
to 14ev with a dual_iso 100/800 shot with virtually no drawback because noise doesn't matter at these settings - with crop iso800 already has noise so it's getting awkward. Also to get a thinner dof with crop you have to carry much more bulk, with ff my compact 70-300L @f4 is "thin enough" for me.
: Not directly sensor-iq related and last, but not least - the more expensive crop cameras have an somewhat ok vf, but with ff you see more of what you're shooting due to the larger mirror which is a reason for ff on its own as in "iq for your eye".
I keep using my 60d because it's better for macro (deeper dof, larger working range) and tripod (swivel screen) as the 100L and 17-40L (@f8+) is sharp enough for crop - and of course for tele shots with 300mm * 1.6x crop factor which makes a decisive difference for wildlife. So I ended up where Canon wants you to end up - buying two cameras :-p