Not much. They both look identical. Even their effectiveness is about the same. But some people will notice the difference for finding the rarer files & also might get faster downlds. Many people think of pro as a donation to the programmers to keep the program in development. Basic can downld from a maximum 8 hosts per downld. Pro is 10. Pro connects to 5 ultrapeers whereas Basic 3. Pro tends to give you turbo-charged connection more-so than basic. People with fast connections will probably notice the difference. But effectiveness also comes down to your abilities to find what you're looking for & finding many sources (hosts) to downld from. People using dial up connection don't need pro since they won't see any difference.