The difference between ROTJ and OWK (clean parts) are:
-Different chest box (I prefer ROTJ because it looks good even when not lit up)
-Different cape (both are meant to have the collar be shown but the chain is on the bottom of the collar for OWK and midway up the collar for ROTJ. So the collar stands up more on OWK. OWK also has a pleat on both shoulders makes it hard to have it looking good in most poses)
-Different mask and dome (ROTJ is probably more accurate to the shape. Though the mouth grill looks a little big on ROTJ. Both look good.)
-Different cod piece (the OWK looks like a big diaper. ROTJ looks a lot better)
I don’t know if the body is different. OWK is probably bigger and using the RO body but I haven’t put them side by side.
Overall, due to the OWK Vader having a dull chest box when not lit up, and a big diaper, and perhaps a slightly less accurate helmet, I would chose ROTJ if I were to only have one Vader in my collection. Which is annoying since it seems that hot toys produced alot more of the OWK Vaders.