Bonaire is what it is. The dive experience is pretty much the same from one diver to the next no matter where you stay, with minor variations such as boat dives, staying at a condo vs. an op like Buddy Dive, etc. The dive sites exist within a fairly short strip and most people dive the same sites, drive the same roads, eat at the same selection of restaurants, etc. That is not a criticism; it works quite well for most people. and I really like most of the dive sites and many of the restaurants. I enjoy Bonaire.
Curacao is what you make of it. Some people have never been to the west end of the island, some people never spend time on the east end. Some people dive once a day and spend time at discos and casinos and playing golf and whatever, while others dive just like on Bonaire. Some people (actually I think a lot of people) go to Curacao and don't dive at all. But because Curacao IS a big island, I am unlikely to even see those people as I tend to stay in the dive areas. I typically see as many iguanas and goats as I do people. Generally, diving Curacao involves more road time. I enjoy Curacao.
Bonaire generally has rocky entries and short swims, Curacao usually has gentler entries and longer swims. Both islands lack many big fish. A number of dive sites on Curacao are at beach areas with on-site dive ops, showers and restaurants, and often charge a small fee, but there are plenty that are remote, much like Bonaire. I dive a number of dive sites on Curacao that are pretty much unmarked and there is often no one else there. The sites that charge a fee usually offer semi-safe parking if you want to leave a cooler or a change of clothes in the truck. I actually like some of the beach restaurants and find it makes a nice SI to have a quick fresh water rinse and then sit in the shade enjoying a mango smoothie and a sandwich while planning the next dive. Next trip I am thinking of going to one I like, doing a couple dives, having lunch and then hanging out at the beach for the afternoon, which is not much of an option on Bonaire.
Whether on Curacao or Bonaire, my typical day is: Get up and eat, load tanks and gear in pickup, do a dive, do a SI, dive again and then either get new tanks and dive more, or relax in the afternoon and do a night dive. On Curacao it is harder to get in 4 dives because driving distance is greater. On the other hand, there are several sites where I will do one dive going left from the entry point, and then the second dive I go right from the entry point. Not a lot of driving then.
I almost never see people towing a dive flag on Curacao, but I think the law requires it. On the other hand if you need gloves to keep warm no one cares if diving Curacao. This is small stuff for most of us.
From a diving perspective (without regard for a scenic downtown, casinos, etc., ) I think they are more alike than different and would tell someone to go where they can get best deal. For most of us, one place or the other is usually a lot cheaper to fly to. If price were the same, I would alternate islands one trip to the next, but for me Curacao is much cheaper so I usually go there.
Really, they are both fine places and I would gladly be on either island right now. If you have been to one and can economically go to the other, I think you should take the opportunity to go and then you can decide for yourself which you like more.