People seem to be a little confused.
iPhone OS X and Mac OS X are the same except they each have unique features. The iPhone has a launch screen and the Mac has a dock, for example, but those are just features and Apple could switch them if they wanted to. Likewise "no local storage" and "have to get apps from iTunes" are not things you'd have to build into the iPhone. They just chose to for the iPhone
So the question is not "which system" but rather "which features" will a tablet include? I think it will obivously be a hybird system between these two choices.
My guess is this:
Start with the current iPhone OS.
Keep the launch screen (or something kind of like it) but put a desktop-and-file system under it that you can switch back and forth from. It's more locked down than the Mac, though. You have things like 'Documents' and 'Music' and 'Downloads,' but not every system file like you see on Mac OS X. You have access to your personal files but not the applications and preference-files and things like that.
Add some kind of application switching feature using a simple gesture, very similar to 'command-tab' on a Mac.
Applications are unique to this device. You'll have to buy the 'Tablet' version. You'll be able to get iLife, Word, and Photoshop, but they're specially made versions for the tablet. Mostly they're the same programing underneath, but the GUI is re-made to work with a touch-screen. You will NOT be using the pull-down menus from Mac OS X like your finger is a mouse. They'll be made to work more like iPhone apps even though they're really more like Mac apps. Apple will have some program that helps you turn a Mac app (and iPhone apps!) into a tablet app in "just a week." They'll bring out some small developer on stage to explain how he did it by himself in 3 days.
So you'll quickly see many of the same apps you can get on your Mac and your iPhone coming out for the tablet, but they will be different versions that you buy seperately. The smart developers will offer a way to buy all 2 (or 3) versions for one low price.
These apps will be available on iTunes, just like iPhone apps, but they will also be available from the 'net, like Mac apps. So iTunes is a safe, easy, and premium way of getting apps, but you won't be locked to them like with the iPhone. After a year, Apple will brag that 90% of tablet users only use iTunes for their apps.
There won't be free-floating windows like on OS X. Developers will design apps to switch between screens you need. Due to the larger screen size than the iPhone, an app can display several 'panes' at once. You can re-size them, but they always stick to each other, just like re-sizing windows in Apple's pro apps like Final Cut Pro and DVD Studio Pro. You'll never see the desktop through an app, you have to specifically switch to it (which will look like expose).
Just a few guesses. I'm just making this up, so the details won't be exactly like this. But I'm nearly positive that "Tablet OS X" will be its own things that's different from both iPhone OS X and Mac OS X.