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Basically with the same photographer, lens, setup, lighting, subject, everything: Can you tell the difference in a portrait taken with a crop vs a FF? I guess another way of asking is, why do people choose FF vs crop for these type of images?
Generally, you can't tell reliably in double blind tests unless conditions are extreme. Extreme conditions include 1) Very large (20 x 30, say) prints, and 2) Razor thin depth of field. The larger the format the thinner the depth of field (when the same perspective is compared), so razor thin DoF is a high sign (but not a sure sign) of full-format.
Now, if you are not doing double-blind and not doing 'blind' testing -- which is to say, if you are testing where the observer knows which format made which, then you can tell subtle positive differences in the full-frame camera. This is the reason photographers who seek the ultimate in quality generally choose larger formats, when other factors permit. Other factors of course include lens selection and ISO capability as well as physical camera size (thinking medium format and large format 4x5 and 8x10).
You can and will read interminable debates about image quality this and image quality that, but nothing will compare to you shooting and printing and evaluating with your own eyes, especially if you do so side by side, adjusting the focal length to make the comparison fair.
Since you can afford to do the comparison by buying, do so. Or at the very least rent.
Nothing less will suffice.