I hate diamond Durant. He's a good slasher but his jumpshot is too inconsistent for me.
Try some things out instead of looking at card stats. Durant is very good on defence for me because his length and quickness on the perimeter.
James has the strength advantage over him in the post on defence. I have no problems with him at 4 in offline gameplay at all. His rebounding is just fine. Only time he was outmatched was in the Allstar challenge by Griffin but I always have a traditional PF on the bench when I lack rebounding and post defence (Ruby Cousins is a beast for me btw).
James is dominant in RTTP because alot of players don't use their starting PF on offense to play in the post. Just for defence and rebounding. With 4 out 1 in you create alot of mismatches on the perimeter with James. Try it out and try to make plays with him, you will see he dominates if used correctly.