We’ve been growing tomatoes at Terra Firma since the beginning, and we started growing heirloom varieties in 1994.  Over the years, we have experimented with hundreds of varieties and settled on just a few dozen — some of each.
A quick explainer:  An heirloom tomato is a variety that has been selected over many years and is “open pollinated”, meaning it has not been intentionally crossed with other tomato varieties.  However, many of the most popular heirloom varieties resulted from an unintentional cross between two different tomatoes.  Open pollinated varieties produce seed that, when planted, will grow into the same variety (at least some of the seeds).
Hybrid varieties are produced when two or more varieties are intentionally crossed with each other.  This isn’t done in a lab, but rather in a field.  It must be done in a way that prevents unwanted varieties from “contaminating” the selected ones — usually by planting the field in an area isolated from other fields of the same crop.
Tomatoes are hybridized for many reasons, but the most common one is disease and pest resistance.  For example, you may have a delicious, beautiful tomato that is severely affected by a soil disease that kills the plant just after harvest begins.  So you cross it with a less delicious variety that is immune to the disease.  Dozens of varieties result, but only a few produce the desired result:  A beautiful, tasty tomato with disease resistance.  You take the seeds from those tomatoes and plant them again, eliminating all the other varieties that might spring up before they produce seeds.  Then you save the seeds from the desired plants and sell them.
We do not grow our tomato seed, but are always searching for good varieties.  We have identified the ones that taste good and produce well on our farm — a mix of hybrid and heirlooms.  Some do better in spring and early summer while others prefer warm nights and very hot days.
In general, we have found that heirloom tomatoes are extremely sensitive to the weather.  The plants don’t grow when it’s cold in the spring, and the flowers fall off when it’s too windy, cold or hot.  The hybrid varieties are also affected by extreme conditions, but less so.
One thing is certain:  tomatoes like the drought, or more appropriately, the weather we’ve had since it began.  Spring storms bring most of the things they don’t like:  rain, wind, and cold.  Heat only bothers tomatoes when it gets over 100 degrees, and so far we have had relatively little of that weather.  It’s been sunny, warm and dry since the first plants went in the ground, and as a result we have a bumper crop on just about every variety, heirloom or hybrid.