There's no functional difference between 1Password for iPad and
1Password Pro apart from the Universal nature of 1Password Pro, but
as you've already mentioned that only really applies if you plan to
ever own an iPhone or iPod touch.
Ideally we'd love to have just one version of 1Password in the
App Store, but because 1Password Pro originally (before the iPad
arrived) has more functionality than 1Password for iPhone it's
virtually impossible to merge our user base.