A few years ago I compared Pro-mix's compressed premium moisture potting mix to Miracle-Gro potting mix and the cheapest potting mix I could find. I also did a box mixing all 3.
All performed well, but not as well as when I used Earthbox's growing media. (which I foolishly dumped into a raised bed, not realizing how impossible it would be to find one of the recommended replacements in my area)
Pro-mix and miracle-gro were very close in performance. I liked that pro-mix didn't have a strong odor and didn't have as many big chunks (wood/bark?) once I broke it up. The downside was that being so well compressed, it was hard to break apart and make fluffy with my arthritis.
The cheapest potting mix had a very strong odor, more big chunks, and didn't perform quite as well as the others. (It performed WAY better than my raised beds, just not quite as good as the other two.)
I got a couple of new Earthboxes this year and will probably need new growing media in 2 of my old boxes. A family member gifted me their unused garden stuff, so I think this year I will compare Miracle Gro Performance Organics to mixing my own.