Ok, here's my take on it as one who has only used 126, 135 and now APS-C...
The difference between APS-C and FF is reasonably close to the difference between 135 and 120/220. You accomplish two basic things with the increase in format size:
a) You have a larger palette upon which you paint your image which allows higher quality enlargements/prints.
b) You get a wider angle of view for a given focal length. As a result of this DOF and other characteristics of the frame change when making "identical" compositions between formats.
I don't think, at least in the digital world, it is a matter of "one is better than the other" - except with regards to the technology behind the scenes (signal to noise ratio, etc.).
With film, the only way to increase print quality is to increase film size. But with digital megapixels (as long as they're clean) really do control how good our enlargements can be. But megapixels have no bearing on the subjective quality of our images, and this is where the FF argument comes in:
At 10ft with a 77mm lens there is a 10 degree increase in angle of view with FF/35mm over APS-C. And while with FF/35mm you might be 6.5 ft from your subject to make that photo, with APS-C you'd have 10ft between you and the subject. That's a pretty significant difference in the "quality" of the image and also in the photographer-subject relationship when for example making a head and shoulders portrait.
I'm not claiming one is better, or more desirable, than the other - I'm just stating that they are significantly different. Someone that shoots primarily macros of flowers may prefer FF as it allows them to get closer and get more of the blossom in frame - while an insect macro fanatic may love the "extra reach" that APS-C provides.
So there you have my late night ramblings whilst scanning 36exp of Neopan 400.