I can't remember who I had higher between Vader and Aja. Both in the same ballpark, around the 20s for me. Two best 'big men' ever. I guess you could say Vader performed well and adjusted to different styles and contexts (and yes, he was good in WWF too) and Aja fell away a bit as a great worker, which I don't know if you can say the same about Vader. Both awesome though and could see arguments for either being better.
Primarily, presence. Vader stands out from everyone around him, you'd put money on him against anyone. Aja is 'stoic', yet she wears goofy face paint and baggy bottoms and is pushed as a monster. It doesn't add up. She's not as 'legit' or intimidating as Chigusa or Hotta and she's not extraordinary or larger than life like fellow monsters Dump and Bull.
And Vader/Sting feud blows away . . almost everything really.
Both of these are correct to put it in my own words
Vader had way more versatility and athleticism an unfortunate point that I think was made strongly through out the process was "We live in the world the way it is not the way it should be" in that people only got the chances they got as such Vader had more chances with more opponents does that mean that Vader was definitely better than Aja? No but, it does mean that we saw Vader get to show more than Aja because as a Joshi she had a much more limited pool of people to work even later one when she branched out more she didn't have anywhere near the impressive held world titles on 3 continents run Vader did.
In-ring Vader had much more varied offense and sold in a way that was more pleasing to my taste I say to my taste because I know some could and would argue that Aja's style of selling was better for a monster character which I don't care to disprove. Over all Vader just felt more game and up to the challenge of working with people more than Aja once again this is a preference to my taste at this point though with Vader clearly finishing higher than Aja I would have to assume that this is an opinion that is shared.
Vader changed what is expect from a big man wrestler in a way that could never been undone he is now the standard that big men workers are held to I just don't get that same feeling from Aja