What exactly is a "genius"? Is it based on smarts, talent, or invention? It's difficult to define. Yet, it is a term that is commonly applied to both Michiru and Mizuno Ami / Sailormercury. Many fans debate over "who's smarter" or "who's better," but no one has quite come to a definite conclusion. But is it really a battle of water queens or natural intelligence vs. learned intelligence (nature vs. nurture).
Michiru and Ami are both considered to be quite intelligent. Although Michiru's IQ is never actually stated, it's usually inferred from her sharp intuition and perception. However, some fans believe Michiru is much smarter than Ami. Many point to the fact that Ami constantly studies and works to get her grades while Michiru is "naturally born" with it; the whole "nature vs. nurture" debate comes in. Naturally, both women seem to have been born with an inclination to learn and adapt. Ami's interpretation is shown through her record-breaking scores on test. Michiru's is displayed through her talent in music and the arts. On the nurture side, both women study in their fields and achieve their "greatness." Many Michiru-fans neglect the fact that she wasn't born just knowing how to play the violin; she must have worked at it and studied somewhere with someone. If you notice, she is always practicing (musically "studying") on her technique, tone, and memorization.
There's also a similar athletic ability inside the water goddesses. In Episode 97, Michiru and Ami race twice in a public swimming pool. Ami even impresses her with her speed and agility. The two end in a tie both times. Now, Michiru does notice - in the first race - Ami's little self-confidence and how she slows down to "let" Michiru win. In the second race, she does swim with her all and ends up breathless. Some can argue that Michiru never raced to her full extent and only wished to build up Ami's confidence, but it's a bit hard to prove. Nevertheless, the general consensus is an equal swimming talent between the two. Episode 170 pitted Ami's racing and running speed against Michiru's. She runs with SUranus to try to find the other senshi. I was impressed myself with the pace she keeps with Haruka. Ami doesn't quite run side-by-side with her but stays at a very distance with her. Yes, she is out of breath when they stop, but she shows that she has some kind of running talent that almost matches Michiru's.
Nonetheless, fans almost always pull out the power card. It's an accepted fact that Mercury is the "weakest" soldier (in terms of offense and strength). But it raises the question: who has the more workwhile techniques. Examining Neptune's attacks, she focus more on completely isolating her enemy and attacking from all sides (drowning, submerging); but, her attacks must hit the target first - and they usually do. Mercury's, however, focus on finding weaknesses and stopping enemy movement (freezing, confusing); yet, her accuracy is not shown on the same level as Neptune's. To me, it's very difficult to compare. I do lean towards the defensive and efficient water/ice attacks, though, but I definitely see the advantages of the "surrounding ocean." It's hard. It's really hard! Basically, I think is just a matter of idiosyncracies rather than outright fact. I mean, if you like the offense, go with Nep; if you like the defense, go with Merc - it's up to your preference. Just remember that both skills are equally important in winning any battle, real or not.
Even through all this debating and genius vs. genius challenges, Michiru and Ami don't seem to compare themselves to each other. There's a mutual respect and probably admiration for each other. Referring back to Episode 97, Michiru comments on how she felt sorry for Ami's self-doubt and even enjoyed racing against her. As soldiers, they've never really fought together, but it probably wouldn't be hard for them to figure out a good plan. Together or separate, "frozen seas" is a deadly combination filled with grace, beauty, knowledge, and know-how. And anyway, who doesn't like water?! ^_^