The "Pro" version is complete, full-featured, and IMO unneccesary unless you're not going to be running it on a Mac. I use "LogTen Mobile" to enter flights and sync them to the Mac later, and it's not nearly as pricey.
Well, what the hell. Looks like they're not selling anything but the "Pro" version any more. They used to have "Student" and "Private" and "CFI" type versions at lower costs with feature sets that each type of pilot would use. $50 for the iPhone version plus $149 for the desktop Mac version is a bit much if you're not a professional pilot (there are a TON of features geared toward the pros).
I guess at this point, given what's out there now, I would probably just buy LogTen Pro for the iPad only - It's with me when I fly, and it's got enough power and screen size to be the sole device I use it on.