Must-do sites: Saba definitely. In St. Martin area Proselyte is the best-known, but Hens and Chickens or One Step Beyond seem to be considered the best sites when available (they can be choppy betimes...).
I did find one website with Anguilla dive sites but haven't found a lot of details abou them to specifically call out one over another aside from the archeological wreck site sounding interesting. See
http://shoalbayscuba.ai/
Regarding operators: My wife and I are headed down early July and have contacted Dive Safaris, Dive Adventures, and Ocean Explorers on Sint Maarten. The following is our impressions as we plan things out - we've had no experience with any of them YET. There's also ScubaFun but they're more on the French side I believe and since we're staying over in the Simpson area we've concentrated on the 3 operators mentioned first.
All three have responded with detailed and courteous emails. All three have some adherents from past threads on this board. Of the three, my wife and I will probably dive with one or both of the first two, but mostly because they offer Nitrox. Dive Safaris also has a specific Shark Awareness Dive (pole feeding to draw them in).
I have to say I was most impressed by the policies and experience of LeRoy French at Ocean Explorers (they take out the smallest groups at max of 8, and their minimum diver number is ONE, so they'll never cancel on you), but they seem to have the highest prices and no Nitrox. Both Dive Adventures and Safari have max 12 (excluding the 'cruise ship' location of Dive Safaris perhaps, plus they have a smaller boat for 6 only) and say they'll cancel if a minimum isn't reached, but the summer is a high season for scuba and neither anticipated that to be a problem. Others have posted here recently that DA did take them out even when they were solo though, as even the guys running the op would rather dive than sit in the store.
Regarding Saba diving again my wife and I are swinging by there myself, so we've done some research and made some contacts but can't report on the follow through. We picked Saba Deep, but really all the operations on Saba are reputable. Dive Adventures says on their website they'll set up a ferry and dive package at Saba for you with one of the operators, but as you mention you're already looking into the airfare that might be a better option - fly over and just book with the local Saba operators themselves. I've seen the same posts you have about the discount fares in cash in person, and was told by the Saba Deep operators there should be a fare as low as $53 round trip available (which is cheaper than the Edge ferry!) which again indicates booking your own is probably cheaper than going for the package setup from St. Martin.
Good luck - please post your impressions when you get back of whoever you chose so we can all learn from it!!
RTRski
p.s. I get the feeling they all know each other on both islands. The guys at Saba Deep did recommend LeRoy @ Ocean Explorers to me, but also said 'whoever you dive with say hi for us'. Since they're all on friendly terms hopefully that means no one has any professional dispute with a particular operators practices, safety, etc. Then again, maybe being friends they wouldn't say so....
If you want something other than an AL80 Saba Deep can accomodate you. If you ask another operator for a different tank they will also accomodate you but they borrow the tank you want from Saba Deep 
