You can see a side by side comparison of the licence types here on the
ClassiPress features page.
The main differences between the Personal Edition and Small Business Edition licence is that the Personal Edition does not allow you to remove the credit links in the footer, does not include the Transaction Logging or PayPal IPN features, whereas SBE does.
The Transaction logging tool allows you to view transaction information for ad purchases from within your ClassiPress admin dashboard.
All upgrades are free, so for example, right now we are on ClassiPress version 3.0.5.2, and when version 3.1 and future upgrades are released it will cost you nothing to receive this upgrade. You receive the upgrade of the licence type you own.
You always have the option to purchase the Personal Edition licence now, and if you decide you want to upgrade to Small Business Edition later you can do so, and you only pay the difference in price.