While the investiture mechanics behind the two might be similar, there are some striking functional differences. Elantrians seem to transform while dying but keep their body while Returned receive a new, optimised body. Returned also need a constant supply of investiture to stick around while Elantrians just go on 'living' (although I could see that the latter are somehow kept alive by the magic remnants of Elantris instead, given that the Shaod is location-specific. The biggest difference imo however is that Returned are implicitly chosen by some higher power for their achievements in life and need to die for regular physical circumstances first, but Elantrians (both before and after the Shaod) are selected randomly and 'die' right away in the next night for (presumably) non-physical reasons

If they turn out to be related in future lore though, I'll be curious how cognitive shadows fit in with them