Pro will connect you to 5 ultrapeers instead of basic's 3. This means that there's the potential for better search results. People who look for rare files seem to find that Pro's a lot better, if you're just searching for mainstream stuff I don't know that you'll see any huge difference.

Pro will allow you to connect to a couple more hosts for download...10 instead of basic's 8 (14 if you have a T3 connection).
Make sure that you're happy with basic before upgrading to Pro

There are fakes & spam around, these can show up regardless of whether you're using basic or Pro. Try using the keyword & junk filters (the junk filter takes a while to train & some people don't like it. I use it).

Check the threads below for more info in these filters & also some general info & tips about searching.

Junk Filter
How to find music(contains lots of tips about using the keyword filter)
Fake files showing up in search results