A ten year comparison of organic versus conventional crop content found that the flavonoid levels in organic tomatoes were almost double that of conventional tomatoes. One of the flavonoids studied was quercetin which has strong anti-cancerous properties and can help with allergies and numerous other conditions.
 
J Agric Food Chem. 2007 Jul 25;55(15):6154-9. Epub 2007 Jun 23. Links