I am literally just debating this exact same question and searched the forum to find your question. Would love to know everyone's opinion. Some have mentioned Mikimoto has crazy markups? From their website
https://www.mikimotoamerica.com/stud-earrings-in-yellow-gold a AAA+ 9.5mm is $21,714.00 USD vs. an A 9.5mm $5,650.00 - Is the AAA+ worth it?
I am a big fan of Mikimoto and they are the only pearls I buy. Here are my thoughts to share...
First, I am an akoya fan, so I cannot comment on their south seas. As I have stated above, for a beautiful glow and luster, nothing beats an akoya. South Seas are large, but the satiny finish is a bit dull to me. The akoyas have almost a mirror glass finish, and honestly, put a pair on, look at your face, and it just makes your face glow (can you tell I really love them?!)
On what grade... I have looked at all grades... my opinion is A (or A1 if available) is the best bang for the buck. First, Mikimoto pearls are already the top 5% of all pearls. So the lowest grade you buy is already in the top tier. Their luster and glow is amazing. I have many pieces, including strands, bracelets, and earrings, and many different grades. I can say from A1 to A, I cannot see much difference. From A to AA, I can definitely see difference but it is not significant. From AA to AAA, I see the most difference. The AAAs are like glass, they are amazing. However, their pricing is very high...AAAs command a very high premium, especially in anything larger than 8.5mm.
For me, knowing that Mikimoto is already at the top 5%, I would say for me.. I like to buy A. On pearl size, I used to think I wanted the 10mm akoya studs. I thought... 10mm, gorgeous, huge, and it is also the size for an akoya. When the SA brought them in, I felt they were actually too big for my ears and took away from that glow/elegant look. So I went with 9.5mm which was perfect. My 9.5mm are As. I also have 8.5mm studs which are AAs. They 8.5mms have a nicer glow, but not by much.
If you are considering a pair of studs, may I suggest you try 9.5-9.0mm and also the 8.5-8.0mm. For most people, I think the perfect size is 8.5mm-8.0mm. I just prefer a slightly bolder stud look which is why I have both.
You may also think about if you want to get other Mikimoto pieces in the future and if matching the pearl size matters to you as it does to me. I like my pearl sizes to match.
Last thought... I would also recommend you look at the overtone. The overtone can be silver or pink. When I started buying pearls, I didn't think it mattered to me so I just focused on the glow, but it does matter to me now. I have a mix of silver overtone and pink overtone pearls. I love how it all mixes when I wear the stands together, but if I had to do it all over again, my preference is for pink overtone. You may have a different preference or none, but you may want to investigate a bit. The SAs should be able to bring in pieces to compare. If you can get to a Mikimoto boutique, it's pearl heaven. They have tons of strands, earrings, everything, and it's worth going just to get the basic 101 on pearls and Mikimotos if you plan to acquire additional pieces.
Long story short... for me, grade A is what I always ask for now.
I hope this was helpful.