I think I agree that buying the whole thing outright from the beginning seems to make sense financially. Also, I believe at some point, I could potentially use the buddy feature, because my 12 year old son wants to get certified next year, and given his slightly scatterbrained nature, I'd like to keep an eye on his levels, too.
I originally was thinking about Suuntos, but they seem to be more expensive feature for feature, and while a third party dive logging software for Macs only works with those, I'm now thinking how little I'd actually use that. I'd enjoy the process more of writing the stuff in the logbooks anyway, and can manually enter my info, or even just use Parallels (the PC emulator), since I won't need to look at it daily.
Now to convince my wife I need to spend $950 on a dive computer! I think it will be time to sell one of my basses first.