A good case partner is much more important, because they can make any case engaging and insightful. When finding the right case partner, pay attention on how well they phrase their feedback.
A good case partner (even if not as experienced as a coach), focused the feedback on the key aspects that actually matter (good, MECE framework, effective use of the framework, focus on what's important, well structured math, etc), and does not get hung up on small, irrelevant pieces of advice they have read in some book ("You have to repeat the case prompt!", "Use the phrases x,y,z",....).
Of course it is helpful to work with somebody who is in a similar stage of the preparation as you (and not significantly less experienced), as not only your case skills will develop during the practice, but also your feedback skills. Ideally find somebody with actual interview experience (i.e. a friend that works or has worked for the MBBs).