Since you’ve been to Bonaire, I’m sure you know it has a road that’s pretty much within a stone’s throw of the western shore from the one end clean to the other. Especially on the southern end, you literally can drive for miles and stop at another named dive site every couple hundred yards. I call it a dive site with an island attached.
The roads in Curacao aren’t nearly so conveniently laid out. Finding and getting to most dive sites takes a bit more determination. The ocean often is out of sight of the main road and some of the sites are 5-10 km driving distance from it.
You can pick up a free map at the Bonaire airport that you can follow and get to any site on the island. Almost every road has a sign and most intersections are marked.
There isn’t a decent Curacao road map to be had on the island (unless Samsom has them in stock, which is rare). The widely-available “Drive and Dive” map shows the general location of the dive sites but far too little detail of the minor (sometimes unpaved) roads you must follow to get to some sites. For that, you need Berndston & Berndston’s ‘city map’ of Curacao (ISBN 3-89707-501-6, available online and _sometimes_ at Samsom). Most roads are unmarked. I don’t know if they were never put up or if they’ve been stolen but this island has far too few road signs. Which is why a vague map is precious little good. You can’t count on the signs to tell you where you are. You need an accurate and detailed road map that you can compare to the roads as you find them in order to know whether you’re on the road to Blubaai or Boca Sami.
In many cases, Curacao’s entries are even easier than Bonaire’s. Curacao is rife with sandy beaches and the beach entries here generally don’t have all the underwater boobytraps of the stony Bonaire entries.
Most of Curacao’s dive sites are shore dive-able but not nearly all. Without actually surveying them, I’d guess between 60-70% are. The remaining 30-40% either are a long, long swim from the nearest convenient point of entry or are offshore from privately held land (no trespassing).
Purely considering the quality of the diving, I’d have to give a slight edge to Bonaire. If it’s an overall 10/10, Curacao is a 9.5/10 (worst case, 9/10). And any place that has diving that’s close to as good as Bonaire’s has to be exceptional.
But diving on Bonaire is just plain simpler. Life there is simpler. There’s fewer distractions and hindrances to your pursuit of diving than on Curacao. So if the trip is mostly about your buddy, IMO, Bonaire probably is the place to go. But if you’re keen on experiencing a whole new set of dive sites, …well, I don’t think you’d be disappointed by the diving here.