60 fps Composition vs Exporting in 60 fps

Entirely depends on what's going on in the comp. This question is very specific. The main limitation would be that certain functions and effects would not correctly calculate motion blur and other secondary stuff tied to temporal sampling because they are not aware of it. A typical expample of this is sudden ghosting on fades because people don't consider that a finer granularity in sampling might require to change the timing. Similarly, some expressions based on time may produce different results to the point of becoming unpredictable. and to add further complication, it's different when rendering from AE's render queue than from AME. Generally, though, since AE is based on "fluid time" and handles the conversion pretty well the impact is minimal on most standard projects. Generally, though, it's simply good practice to design to spec and given that a lot of stuff can be checked beforehand by nesting comps with different framerates/ creating duplicates and changing the comp settings it's perfectly manageable. You just need to put in the effort.

Mylenium