I was wondering the same sort of things. I registered Express Accounts because I don't like running unlicensed software but what benefit did this give me apart from the warm fuzzies ? I can't see any reason to upgrade.
Also, looking at Express Invoices, measurements can be set to default to millimeters whilst Express Accounting cannot - it is set to inches.
Another question to add to your one too - what functionality does adding Express Invoices to Express Accounting give me?
Very little information readily available to work out the answers. It's an expensive add-on that doesn't seem to giver me any real added functionality.
Thanks and regards,
Chris