Here's a link to my trip report from March 2010 to Bonaire, which contains a link to the trip report from a prior trip in '09. These 2 trip reports use a lot of photos to illustrate what it's like to go, and should help you get an idea what a trip to Bonaire will be like.
I've visited Aruba one day while cruising; haven't dove it. Have seen people talk about it on the forum. In a nutshell:
1.) Aruba's got gorgeous sandy beaches and Bonaire has nothing close.
2.) In Aruba you will probably pretty much be going on boat dives - my experience with boat dives elsewhere is something on the order of $125 per person, boat takes you out, you dive twice, each in a separate location, and they bring you back to the dock.
3.) Aruba has wrecks. Bonaire isn't as 'wreck famous,' but you can dive the
Hilma Hooker, and if you like small, the
Bakanal and
Our Confidence.
4.) Aruba may have more 'entertainments of civilization' topside aside from diving - such as casinos.
Now, Curacao vs. Bonaire is a more complicated issue, and there are threads on the forum discussing and debating that in detail. In a nutshell, Curacao is larger, also has shore diving but isn't as 'dive focused,' and either can be a lot of fun. Neither is known for much larger sea life aside from tarpon, sea turtles & the occasional barracuda or big green moray, so don't expect sharks, dolphin, manta rays and such to be likely to be seen.
Richard.