I will agree and disagree with the above posts, all at the same time!
Bonaire and Roatan are certainly in my Caribbean "Top 5". Sooner or later, you're going to want to do them both... repeatedly!
Bonaire and Roatan are both well outside of the path of hurricanes. True enough, nature has and may visit both islands with a huricane, but a simple glance at any historical chart of huricane paths- you will see that neither island is much of a target. September? I could think of better times to go to the Caribbean, but you have selected two of the more 'huricane proof' locations!
Roatan, being recently in hi-speed development, it had started with a strong established 20+ year base of AI resorts that cater to US Diver requirements and mentality. As the island has exploded in the last ten years, more operations have sprung up that are set-up for the European crowd. Entirely different programs for people with different demands and needs.
Bonaire has been around a long time. It was slowly nursed into its status as a dive mecca by American divers. The place thinks, operates and moves to our tempo. True enough, lately Europeans have visited in droves (Note the KLM 747 sharing the runway with the goats!), but the die has been set on Bonaire. You can have all the diving you want, if not from your resort, then via shore diving.
Just as a general point of information, if you visit a place that carters to Europeans, you will have to struggle to get what you might think is a requirement. 3 dives a day can be problematic (much less 5 a day)at such locations. Notably Red Sea, Maldives, Mediterranian and various other pockets located around the world that welcome divers who speak German, French, Italian, Spanish and moree! It isn't bad- it's just what it is. You have to understand.
Bonaire is a delightful place, any number of pleasant comodious resorts with an accent on your automotive mobility and access to numerous shore dive sites. The diving is good for beginners as the walls are vertical and there's lots that is easy to see. Every property there is a dive resort. There are 20 restaurants that I have been to and waddled away smiling.
Roatan? You would have to be nuts to drive a car, besides- there is simply no need or use for one. 99.9% of all diving is done by boat. There are three restaurants, arguably 5 at most, that are worth bothering with. The diving is somewhat similar, but really biased towards the diver with super buoyancy and observational skills. There are some resorts with diving, there are a few real dive resorts. Many divers do not like it because a) they have been to a bad operation or b) they do not have the patience or skills to observe the small creatures for which Roatan is truly known.
Decide what kind of experience that you are looking for and then book accordingly. You will hear miriad opinions about various resorts on Roatan, but largely driven by single visit or narrow view observations.
In Bonaire, you get a room to sleep in, you can always drive to dive. Pretty much so can't go wrong. No resort in Bonaire has a shore dive worth much, better for night diving.
In Roatan, depending on what you need and expect, selection of the accomodation (be it an All Inclusive Resort or backpacker paradise), the selection of the accomodation and its access to dive-op is key to the process.
In Roatan: What quantity of diving do you desire? Are you happy with backrolls off of a small boat or were you looking for handicapped access? Do you want 5 a day with an included night dive each night- or would you rather buy ala carte at $30 per tank. Lots of choices for everyone... just not a lot of mobility.
Your "solo" requirement does mean one clear thing- the cost of your rental car in Bonaire, a true necessity there, it will be split... one way.
You are going to do both places sooner than later.
