Howdy Dimondalebergs.
The saying about gmo being more productive than heirlooms is vastly generalized.
Main thing is, with gmo things all patented up by a company that hires lawyers and buys politicians, you don't get to save seed from them, and if you do, you''d be likely to have a whole world of lawyers against you. Generally gardeners are opposed to that kind of thing, but a few seem to like it. Those who don't mind having roundup by products to feed their kids.
Anyway, heirlooms are a great way to go. Those seed companies mentioned already are good for sure.
Another one is Bountiful Gardens. They do a lot of selections for growing under organic conditions, so generally their varieties carry pretty good resistance to disease and pests.