Pro will connect to 5 ultrapeers instead of basic's 3 so the potential's there for better search results. People who search for rare & unusual files seem to find Pro much better that way.
Pro will let you download from 10 hosts compared to basic's 8 (14 if you have a T3 connection) so the potential's there for faster downloads too...depends of course on how many hosts you download from & what their speeds are

If you have a very slow connection, then honestly you probably wouldn't see much difference.

A lot of people see buying Pro as a donation towards LW's future development.

Use the forum search & you'll find this question's been asked often, have a look through older threads for more info