You really don't have to go for the 80 if you don't need it. It's an extra $50-$150 that might be better spent elsewhere.
That said, there are some advantages: namely, that it's thinner than the 30, has more space, and the incredibly fun and useful Zune Pad. It's a Godsend for people like me with large music collections. You really have to ask yourself how much you're going to need it. If you have a smaller music collection, I'd definitely go for a 4 or 8 GB model, as it's got the Zune Pad and tiny size down pat.
Reasons you'd want to stick with a 30: cost, obviously, is a huge one.
What I'd do is head to your nearest Brick-and-Mortar retailer and test drive it for yourself. Size up the 30 and the 80. See how they feel in your hand, and whether the smaller size and decreased heft is worth it. Are you going to keep it in your pocket? Do you need a case?
With the 30, the answer is almost assuredly no. The tougher exterior is scratch-resistant and it comes with a handy-dandy felt case.
Onto more important issues: I haven't had any problems with the Zune2 software. Lots of people said version 2.0 screwed up their tags, but that appears to be fixed now. The software is stable and useful, if somewhat feature-bare. Test-drive it for yourself by going to
Zune.net/Setup.
As a former user of Creative products, I can't stress to you how much it sucked. The devices are hefty, pricey, and somewhat difficult to use (not terrible, but not very intuitive either). They're mostly aimed at the enthusiast set, though not completely. They also never update their firmware, and have a history of taking out features (FM recording, e.g.). Don't know much about the Sansas from personal experience, but they seem to be a good middle-ground. The Connect can wirelessly download music, and the price is reasonable enough. Check around for reviews.
The 30 and 8 are MSRP'd the same because 30 is last generation, and 8 is current. It's simple rules of technology. As I've mentioned above, the 30 lacks a few of the niceties of the current generation, but has all the same firmware and software. It's really your call.
Let me know if you have any further questions.