Have been on the Nekton three times and Mike Ball once and have booked Peter Hughes for December - won the DAN auction
Love the Nekton boats - I get seasick and have even been close to being sick on the Nekton twice, however if I had a choice in any kind of weather I would choose Nekton - it is a VERY stable platform for diving. The other boats will have me on the edge pretty quickly if I am not careful; on the Nekton boats I don't even think about it unless the weather is really bad.
The Nekton boats are very spacious, hard to imagine until you get on one, but 32 people don't come close to filling it. The top sun deck is roughly eight times the size of the top deck on Sun Dancer II. If you want to get away from people - no problem.
The dive deck is designed to dive from - not a converted yacht but designed to be exactly what you need to dive from. Again until you try it, it is hard to imagine the difference.
The only time I have been crowded on a dive site was in Australia diving a pinacle - 18 divers all ended up at the top at the same time - it was a very small site, but also one of the best - nobody wanted to get back on the boat. In Belize, which I have done twice with Nekton, you will not be "crowded" at the dive sites.
As I understand it Peter Hughes and Aggressor offer a dive trip that is more similar to the Mike Ball experience. Better food, wine/beer at dinner a more "personal" luxurious trip. The Mike Ball boat was all of that - and I expect that Peter Hughes and Aggressor will be similar. However I find that the only real difference for me is the food. Nekton is good food and plenty of it, Mike Ball was gourmet food. A significant difference and as a non diver this may be an important issue - as a diver I am just as happy with the Nekton fare. As a diver I want to do night dives and so wine/beer at dinner is irrelevant, as a non diver you may want to bring some extra beer/wine along if you choose Nekton.
Nekton does not do the Blue Hole - both PH and A do. I would rather do another wall, but that is personal choice. Once I try it I may change my mind.
Again Mike Ball proported to be more luxurious and personal trip. I have been on both Sun Dancer II and the Aggressor boats and they are more luxurious as was Mike Ball, more of a "yacht" feel. Nekton is more a "working" boat. I prefer the working boat - more space works for me YMMV - if this is a dream vacation the "yacht" feel could be important.
The "personal" touch is how the staff treat you. Mike Ball was great, the Nekton staff was just as good, from this standpoint I doubt that either PH or A would be better as it would be difficult to figure out what "better" might be!
Re rudeness on the dive sites - I did witness a pissing match between the Aggressor captain and the Nekton captain. The Agressor was tied up to a bouy that the Nekton was scheduled to use. Took about half an hour but the Aggressor finally moved away - from that I take it that the Nekton was in the right and the Aggressor boat was in the wrong. However, I do not know for sure. I do know it cost me one dive that day so I was not happy. The Nekton is a bigger boat with a MUCH larger profile in the wind so I gather it has to be more careful where it ties up.
Bottom line is that any of the three will be a great trip. Nekton is cheaper, not as luxurious, a better dive platform, less likely to get you seasick and much more spacious.