My general suspicion and one reason I start all my own seeds is that the store bought plants are over-fertilized. If you think about it, the store will have more success selling huge lush green healthy looking plants, and why wouldn't they, the average person assumes the plant that looks so healthy IS so healthy. But the reality is that the roots are never encouraged to grow because they are #1 in a pot and #2, given ample water and nutrients, so they only do what they need to. Since the water is so readily available (most water daily) and nutrients also, the roots grow but not at a comparable rate with the top growth. When you put the plant in the ground, all of a sudden, the roots are not getting the same amount of water because hopefully you are not watering daily, and they are not getting a super concentration of nutrients, like the nursery was giving them. Thus, the plant tells the top groth to halt, while the roots grow like mad to catch up.

Now on thew other hand, if you raise your seedlings the right way, with a little tough love. you let them dry out almost completely between waterings, and you only give them a couple tiny sips of fertilizer so they stay green and don't get too big for their britches. Wehn you plant them in the ground, the roots are already at a good ratio to the top growth, and they are going to go right out and look for more water and nutrients since you have been holding back on them. Now the top starts really taking off only after a day or two, and the plant grows in balance and give you a strong healthy plant with proper watering and care. Again, letting it get dry between waterings so the roots keep growing and getting bigger and bigger and deeper and deeper. And unless your soil is complete sand, you really should only need to give them a light fertilizer boost sometime within the first week after planting them out. After that, let them do what plants do and go find their own nutrients. If you are mulching with compost and organic mulch like grass and leaves, your soil should be pretty good at providing them what they need.

CH

Keep in mind you are comparing totally different varieties with very different genetics. Those genetics play a big role, not to mention variables in growing conditions.

Dave

The store bought variety was Cherokee Purple, so it was essentially similar to the other 8 Heirloom varieties, at least as much as heirlooms can be considered "similar" it's not like I had planted some Patio Plant, and compared them to some Cherry's I was growing.

My apologies if your comment was directed at the OP, but even in his case, under almost any circumstances, sungold should have tomatoes on the vine before Brandywine.