This is a myth. They usually taste worse, because they've been bred to grow to a uniform size, to be pretty, and to have a long shelf-life.

As far as nutrition is concerned, it only seems this way because they're pumped full of water and end up tasting just like bags of mildly tomato flavoured water.

If you take a heirloom variety and a mass-produced hybrid variety and grow them in the same good soil in full sun, their nutritional value should be about the same.