Count me in with the dive boat first, park second. Dive boats are just so much less WORK! On some, you can board the night before, get a good night's sleep, and wake up right before you're at the dive site. Get in 3 or 4 good dives, climb in a bunk and go back to sleep - and wake up at your car. Don't have to go hunt for food, don't have to carry your tanks to the fill station, don't have to deal with draggin yer arse to the ferry terminal at oh-dark-thirty...etc.
For me, the motivation for choosing one or the other is completely dependent on the situation. If I'm just looking for a good single day diving, I'll choose the boat every time due to the convenience factor. It's not the money. As Grumpy points, out, parking + ferry + cab + fills + food ends up being more than a boat. But even if the boat was more expensive, I'd choose the boat for the convenience every time.
As much as I love our dive park, it's just such a hassle that I have to have a very good reason to go there...e.g. a class, or an event (such as our upcoming DWAD day).
But on the other side of things, diving the park can mean more total dives...provided you take the earliest ferry in, and last ferry home. You can do as many dives as your NDLs allow, as opposed to boats where you are limited to 3 or 4. Plus, you are guaranteed a good dive each time, as the park is so awesome. Whereas on a boat, you might get stuck at a crappy site (especially if you're on a boat with students). So if I was looking for a really full day of lots of dives, I would choose the park.
I would also choose the park if I wanted to spend a whole weekend focused on diving. If I book a room in Avalon I can dive all day, get in a couple night dives, dive the whole next day...way more dives than a boat.
So there ya have it.
