Think ONLY about frame size...about 15 x 22 for APS-C, about 24 x 36 for FF digital. The two bodies are truly different format cameras, and one ought to use the FL which is appropriate to the format. So if you shot 50mm FL as the 'normal' lens on FF, you would use about 31mm as the 'normal lens' on the APS-C. Or if you used 100mm for portrait on FF, you would use about 65mm to shoot a portrait from the same camera position with APS-C.
If you did that, the same amount of scene would fill both frames, and you would have to magnify the FF image by about 8x to make an 8x10" print, and you would have to magnify the APS-C image by about 12.8x to make the same 8x10" print...so the APS-C print would a a bit lower in quality due to its greater magnfication! In practice, you probably would not notice the difference at 8x vs. 12.8x. But when you make a really big print like 20x30", there is a considerable and noticeable difference in 'about 20x' vs. 'about 32x' to make the same final print!