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This is a bit complicate, and unfortunately some want to try to reduce it to "A is better than B!" or "B is better than A!" simplifications... so I chose my words very carefully in that first post so that it would only say what is true and not debatable. The context means more regarding this question that trying to come up with some simple rule about the relative quality of crop and full frame in this situation.
Here is what we know:
- If you were to put lenses covering the same angle of view on a cropped sensor and full frame camera and those lenses produced identical lens resolution, the full frame lens/camera system would be capable of producing a "sharper" print. Lens resolution is often described using terms such as "line pairs per millimeter" - which is a bit more complex concept than it sounds like, but I'll leave it this way for now. However, the larger the sensor or film, the "more line pairs" that can be captured in the image. Using made up numbers, let's say that we use lenses with 20 line pairs per millimeter (20 lp/mm) resolution two cameras. Let's say that one has a sensor that is 20mm wide and the other has a sensor that is 40mm wide. The smaller sensor can theoretically resolve 400 (20 x 20) line pairs while the larger can resolve 800 (20 x 40) across the frame width, and that is what determines the better resolution potential of the larger system.
- A simple way to distill this is to realize that a particular lens resolution results in higher image resolution as the frame size increases.
- If you compare a APS-C (or "cropped sensor") size image from a cropped sensor camera and a crop of the same size taken from the middle of the frame of a full frame camera, there are essentially two possibilities that we could be interested in. (There is a third possibility, but I'll ignore it since it is very unlikely.)
1. If the two sensors start out with the same number of megapixels (e.g. - the same number of photo sites), when you crop the full frame image down to an APS-C sized section, the sensor resolution of the image is more limited in the case of the cropped full frame image, and its potential resolution is diminished. (In the real world, it might not make a difference unless you print quite large.)
2. If the full frame sensor starts out with higher photosite density such that an APS-C size crop from its center ends up with the same number of photo sites found on the cropped sensor, the image quality should be equal. (This is a pretty unlikely scenario, though with 36MP full frame sensors now on the market, we can imagine it.)
When you are trying to parse out this choice between a full frame sensor and cropping versus a cropped sensor camera and using its full image, the "right" answer depends on a lot of stuff.
If you are only interested in filling the frame with the subject when you use your longest lens, and you are not all that interested in other kinds of photography in which you don't do this - say landscape or portrait photography at more typical focal lengths, then you fall into a special case where you might as well get a cropped sensor body. You'll get more "reach" (e.g. - ability to fill the frame of your camera) with a given lens, and you might be able to use less expensive long lenses.
If you are very critical about image quality - perhaps you frequently make very large, high quality prints - and you do not just shoot at the extreme ends of your long focal length lenses, you have a trade-off to consider: are you willing to give up image quality potential in your large prints for the ability to fill the smaller frame more without cropping? Only you can answer that.