One tank vs. two tank also depends on what shop you are diving through and where you are diving.
First, depending on the shop you use, some only offer one tank dives, others offer two tank dives.
Second, how far the shop is from the sites you want to dive might be a factor. This hasn't been an issue for me in Bonaire, so I'm just assuming...
For example, I'm using Bonaire Dive and Adventure (BDA), which I believe only offers one tank boat dives. Almost all of my diving will be from shore, but I may want a day's worth of Klein (which is mostly done by boat). I don't mind only having one tank dives as BDA is so close to Klein (I think within about 5 minutes by boat) that they simply take you back to their dock for the surface interval. So, if you want a second boat dive, you sign up for two one tank dives. The only difference between that and a "two tank" dive is you get to take your surface interval on land!
If you were staying on the south side of the island and wanted to go to Klein, you may want a shop that does two tank dives, instead of two longer boat rides... I'm willing to bet that the shops that are further from Klein are more likely to offer two tank dives for Klein, but I can't say for sure.