As long as you're grocery shopping in the United States, it doesn't really matter. I've spent most of my life in suburban and exurban California with several years each in New York, Miami, and Seattle so I'm no stranger to farmers markets and co-ops. The difference between homegrown and farmers market is much greater than the difference between farmers market and supermarket.
If I had to quantify it, I'd wager that fruits and vegetables from farmers markets are from 10% to 20% better than what you'd find in a supermarket. Co-ops can be as much as 10-50% better but growing it yourself with proper varieties is easily 200-300% better, especially here in California. Tomatoes with so much flavor that you can taste them through a tablespoon of ranch dressing (not that you'd ever need anything more than a splash of olive oil and a dash of salt with home grown vegetables).
If I had to quantify it, I'd wager that fruits and vegetables from farmers markets are from 10% to 20% better than what you'd find in a supermarket. Co-ops can be as much as 10-50% better but growing it yourself with proper varieties is easily 200-300% better, especially here in California. Tomatoes with so much flavor that you can taste them through a tablespoon of ranch dressing (not that you'd ever need anything more than a splash of olive oil and a dash of salt with home grown vegetables).