Of those on "b", at least 20% will upgrade within 3 months of iPad 2 release date. Why? Because, first, Apple will advertise the hell out of it and, second, there will be a humongous surge of posters on this site writing on how awesome iPad 2 is, no matter how significant or not the new features are. Tempting, right?
Bottom line is I am expecting no more than 25% of current iPad owners passing on iPad 2 and waiting for iPad 3.
My take...
(percentages above are as valid as the rumors of iPad 2 specs)