The pelagic dive is very different and something to experience... If you enjoy macro diving and want to see tiny alien-like creatures, then you should try it. With that being said, you have to be very comfortable in dark water, an endless bottom and have great buoyancy. The water can also get pretty choppy at night.
If you can squeeze in a Long-Range dive day, I highly recommend it. These are smaller and faster boats that take you to places that the morning boats can't make it to because of the distance. They are more expensive, but worth it, IMO. Lava tubes, unique (hard) coral gardens, structure, and always a chance to see something big.
Shore diving is also another, much cheaper, option. If you want to go that route, I would recommend hiring a dive guide or company that does shore diving.
I also must add, if you come to Kona expecting beautiful colorful Cozumel reefs and giant barrel sponges, you will be disappointed. Since Kona is on the steep slope of a volcano, at first glance, the underwater world might looks bleak and "dead" to some. However, there is some incredible marine life here, including species that are endemic to Hawaii, and some that are endemic specifically to Hawaii Island. Also because of the steep dropoff, we are fortunate enough to see pelagic animals quite often. It is not uncommon to see sharks, dolphins, eagles rays, mantas, turtles, endemic fish, 5-6+ varieties of eels and a plethora of nudibranches all in a handful of dives.
Another activity that is offered here, depending on how comfortable you are in the open ocean, is an off shore tour spending the day looking for mantas or pelagics. You never know what you will run into out there! Unfortunately, the whales are already starting to leave and will most likely be gone by your arrival, so whale watching probably won't be offered by then.
Here is a video of blackwater diving (be sure to watch in 1080p for best quality):
And here is a video from one of the long-range days: