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What's the ground soil like where you're at? Sometimes it's better than store bought stuff. If it's not good and you want to buy soil then don't buy any bullshit like miracle grow or kellogs, it's worth it to spend a little more on something good.
I've never used coco but as far as I know it's just a neutral medium, people use it when they want total control of the amount of fertilizer/nutrients their plants get. I personally wouldn't recommend it for plants in the ground.
Edit: I've never used promix either so I can't comment on that.
Don't think you can go wrong with promix. Mycophile is right, coco coir is much more neutral and will take more additional nutrients than the promix which you can probably grow start to finish in without any additives. If your native soil is half decent consider doing 50/50 of it and the promix, the price of soil mix can sure add up quickly.