This is before the hurricane, so everything might be very different now.
I have around 40 dives as St Martin/Sint Maarten and 15 in bonaire. Different dive sites. St Martin/Sint Maarten is all boat dives, bonaire is all shore dives. There are plenty of things to see in St Martin/Sint Maarten. Some sharks, wrecks, and plenty of boring underwater sand, but when your are on a nice site, it is very nice.
If I had to pick, I would go back to St Martin/Sint Maarten, Bonaire(I felt) was all very similar. St Martin/Sint Maarten you can divesdifferent types of senery, which is nice, but you need a boat.
If you go to St Martin/Sint Maarten, make sure it is not a cruise ship dive boat. You want to get away from the Harbor and explore the island.
Check out the pictures and then ask questions.,
http://www.mwdives.com/2011 Sint Martin/index.html#img=2011-11-19 St Martin - Porpoise - That Confounded Bridge_01.jpg
http://www.mwdives.com/2008 Saba - St Martin/index.html