Hypothetical Situation. Kushina has mastered and (also) became friends with Kurama like Naruto, and they spar out.

Intent is to incapacitate or knock out the opponent. Here's an overview of the subject and each of their disadvantages.
Kyuubi Kushina-
Older... more battle smarts/experience.
Capable of breathing with destroyed lungs and holes in the center of her body.. not that that would be helpful or anything, but it's worth mentioning to say she has an incredibly powerful will, and is capable of accepting pain, very well.
More than likely has a larger arsenal of attacks, being older and a pure blooded Uzumaki. She is probably very skilled with at least 15+ different seals, compared to Naruto's different forms of the same attack; Rasengan and TBB fusions.
Same speed as Naruto.
Let's not forget that she can use clones; just not shadow clones.
Can use TBBs.
More chakra control.

Kyuubi Naruto-
Younger. I won't say he has more energy and is more youthful for 2 reasons; they are both still young and energetic characters, there was no way Kushina was over 30. Plus, energy wouldn't mean a thing since they're being backed up with Kyuubi modes.
Same speed.
Different forms of Rasengan powered techniques.

As we can see, Kushina has more advantages here. And feel free to contribute to either one if I'm forgetting something. I just wanted to make this because people seem to overlook Kushinas jinchuurikiness.