A recent thread on heirloom tomatoes showed that there is quite some confusion about this subject. Also there is a tendency to take up positions on the topic that are not necessarily supported by fact. To start with heirloom and hybrid are not two mutually exclusive categories that together describe all edible cultivars. It may be useful to speak of commercial and home varieties but in this we should distinguish between the situations where seeds are selected for either commercial or home growing and seeds that are supplied commercially or saved at home.
To me 'heirloom' means an old open pollinated (true breeding) variety. It often also carries the connotation of being handed down through personal contact, that is being passed around. There are also many commercially produced lines which are open pollinated. Since the commercial lines all had heirlooms as ancestors and since true breeding commercial lines get grown for seed and subsequently passed around, often without acknowledgment of their origin, the distinction is not that useful. Of course there are companies that specialise in selling heirloom seeds but the idea of passing them around amongst friends seems to be important to some people. To me a better distinction is to consider the reason why the cultivar was bred and what characteristics were enhanced or reduced.
A hybrid is a cross between species (horse + donkey = mule) or between cultivars. For edible plant growing we usually mean the latter. Hybrid cultivars often are fertile and true breeding. Hybridisation can be, and often is, used as an adjunct to selective breeding. Selective breeding has been going on since agriculture was invented (say10,000 years) it is the major force that has produced all the edible cultivars that know today from ancestral types that were in many cases barely edible and much less productive. Selection has improved yield, flavour and size, reduced disease susceptibility and many other benefits. Hybridisation also can have benefits whether it is simply combining the good features of two different lines or generating hybrid vigour. The latter effect is an improvement that can be observed when crossing two lines. Not all hybrids show this vigour and it is not always very significant.
Some hybrids are called F1 hybrids. The 'F' stands for filial, that is child. So an F1 is the first child generation, an F2 is the second child generation etc. F1s *may* have a special property of being heterozygous depending on how they are bred. Of course in any breeding situation you can call the first generation the F1, it doesn't need to be a hybrid.
Greatly summarised plants (and us) have two copies of each gene (we get one from each parent) there may be many versions of each gene in the gene pool and in general all possible pairwise combinations are present in the population. So you cannot be sure which characteristics an individual has unless you have bred out all the alternatives, that is you have reduced the scope of the genepool so that only one gene is available for the characteristic, the one you want. If for a given characteristic the two copies are different genes then that is heterozygous, if the two copies are the same it is homozygous. The true breeding cultivar would be homozygous for that characteristic, so each parent necessarily passes on that gene and so the offspring will necessarily have the same gene.
However if instead of breeding the same homozygous line with itself you breed two lines each of which is homozygous but different the F1 will necessarily get one of each type from each parent and be heterozygous and so not true breeding. This is the situation with commercial lines that are sold as F1 hybrids. There are some good reasons for doing this from the point of view of the consumer, some F1 hybrids are superior in some ways. There is also a reason for doing it from the point of view of the breeder, the grower is tied into buying their seed every year as seed saving is useless because the F2, f3 etc can not be the same as the F1.
So what are differences between commercial and home varieties? The difference is mainly in the selection of characteristics that have been used in their breeding. People shop by appearance. In the supermarket large will be bought before small, good colour before poor colour, clean before blemished and flavour very often has nothing to do with it. So what have plant breeders done for the supermarket growers? They have produced lines of edibles that are good looking and durable. You can add into the equation that in some cases produce is machine picked then they have also bred into those lines the property of bearing abundantly over a short period of time. The commercial grower wants to get rid of stoop labour going along the rows selecting the produce that is ready by hand, when the beans are ready the picking machine goes along and harvests all of them in one go. Sadly in selecting for supermarket qualities taste has been relegated to a secondary consideration, this is not an evil plot by dark forces but a continuation of selective breeding that has been going on for centuries constrained by modern marketing and transport.
The home grower may want quite different characteristics. When you grow your own you can select for taste and you would probably prefer a cultivar that bears over a long period of time rather than have a glut and you don't care much if the produce is durable because you are not sending it 1000km to market but taking it into the kitchen. The home grower is also interested in suitability to their conditions and variety. Some grow 6 kinds of tomatoes because some are better for salad or bottling or paste and it's nice to have a choice of size, shape, colour and taste.
So what should you grow heirloom or hybrid? It's a meaningless question, they are not strict alternatives. What you should grow is the cultivars that suit you, the ones that have the characteristics that you want for your situation. These may be available from friends or seed companies and they may be "heirloom" in the sense of being identifiably old strains that have been around for centuries or they may be new varieties. If you particularly want certain F1 hybrid characteristics you might be happy to buy your seed each year, if you don't really want that or even know what it means then save your seeds and your money. It is better to be eclectic instead of religious about growing.
David