The table at http://www.red-gate.com/products/sql-development/sql-prompt/feature-matrix
explains the differences between Standard and Pro, but you're right, it's not very easy to find from the features page. I'll get in touch with the team responsible for the website and see if we can get that improved.
The purchase price entitles you to all upgrades with the same major version number; as you say, if you buy 5.3 you'll be able to upgrade to any 5.x versions we release, but not version 6. There are also various Support and Upgrades packages available that entitle you to all upgrades (including major version upgrades) within a given time period.
It would probably be best to contact sales for more details on all of this; you can arrange for us to call you at a convenient time using this
form, or you can e-mail firstname.lastname@example.org
Hope that helps,