I did an experiment this year with starting most of my seeds in DE b/c It looked cool and I wanted to try it. Overall the DE worked well as I had no damping off and germination was very high, although I haven't had any issues with Metromix before either. Here are some pics of the difference in plants side by side.
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As you can see the metromix plants look much more healthy and happy. What's more the DE plants are 2 weeks older than the MM plants. I did fertilize the DE once a week, at first with urin and later with 1/4 strength fish emulsion+kelp.
Overall I feel DE is a good medium in certain situations where you have trouble with dampining off, but I will be sticking with MMix from now on.
Just to add, I have found that watering seeds in with a weak fish emulsion mix at planting speeds up germination and increases vigor. I had bell peppers up and with their second set of true leaves starting to poke out in 13 days in Metromix with fert added, the ones I didn't water with fish poop are I'd say around 3 days growth behind. Experimenting is fun
