The main difference, as I see it, between 5" and 7" tablets is that I can usually hold a 5" tablet in one hand. I've found that I need to prop a 7" tablet on something; they've always felt too unstable. I also like to put down a 7" tablet so I can use both hands (or at least use more fingers). A 7" screen is big enough that using both hands is more comfortable than suing a single hand.
If I have to stop and put down a tablet then it's not very mobile anymore. I'd rather go with something larger, which would give me more screen real estate.
Now, the one downside of a 9" or larger screen is weight. It doesn't bother me. I carried a 9" Novel with me at CES, and the weight wasn't a concern.
As I see it, a 5" tablet is closer to being a PDA or smartphone than it is a tablet. Yes, I know what everyone calls them, but I'm looking at how I've used them. The Galaxy S Wifi is the next evolutionary step of a PDA, I think.
What's your opinion?