There are two main factors to consider when choosing between organic and regular produce, namely pesticide residues and nutrition.  Information appears below.  Freshness and flavor are important, but these depend on more than how the crop is grown and when it is harvested.  I recall reading somewhere that vegetables tend to be higher in trace minerals than fruits.  If correct, this means that organic vegetables tend to be more important for nutritional content than organic fruits.

It is sometimes argued that organic food is more expensive.  This may be true pound for pound, but that is misleading since it does not count health and medical bills:  The nutritional benefits (see below) can lead to better health;  pesticide residues may cause health problems in the long term.

• ANTIOXIDANT  levels in crops have fallen 50% in the past 25 years due to changes in farming and processing (Nutr. Rse. 96:16 quoted in Alt. 11/02 p.132).  Perhaps fresh organic foods would have "normal" levels, but I've seen no information on that.
• VITAMIN B12  occurs in bacteria and can be obtained from eating ruminants, unwashed foods, and nodules of some root vegetables.  The nodules are more likely to be found on organic root crops and are lost when peeling.
• MINERAL CONTENT  may be the most important nutritional difference between organic and regular produce since heavy use of fertilizer inhibits absorbtion of some minerals, which are likely to be at lower levels to begin with in soils that have been abused.  This may be caused in part by the lack of beneficial mycorrhizae fungi on the roots since high levels of fertilizer tend to kill them.  Standard diets tend to be low in various minerals, resulting in a variety of problems including osteoporosis and reduced immune system.
        The Baer report (Rutgers Univ., 1984) "Variations in Mineral Contents of Vegetables" contained information on organic versus regular crops.  The Macrobiotic Cancer Prevention Cookbook excerpted information on five vegetables.
  Entries are levels in regular crops as a percentage of levels in organic crops  Ash provides a measure of the amount of major minerals present, but not trace minerals which I've listed separately.  I converted actual amounts to percentages and rounded.  Since I rounded, a zero does not mean total absence in the regular crop.  Excluding boron, only two trace minerals entries had high percentages.  These were due to low levels of the mineral in organic cabbage, not high levels in regular cabbage, and so are not significant.