Hmmm....well, Guillard's F/2 (formula diluted by half, one part water:one part formula) is a scientific staple. It was also designed specifically for microalgae, something that Miracle Grow can't boast. Most of the bottle formulas are also sterilized, so there's no chance of contaminating your culture with something like bacteria like with Miracle Grow. I don't believe there have been any studies on uptake of MG nutrients in comparison to Guillard's (i.e. are there leftover elements like phosphorus? potasssium? nitrate? trace elements??).
That being said, I'm fairly certain MG is far more accessible than Guillard's. Not that Guillard's is hard to find nowadays, but I doubt you can wander into an aquarium shop to ask for microalgae ferts, like you can with a gardening store to ask for MG.
It also depends on what you're trying to culture. If it's just plain old "phytoplankton" then MG could work. If it's something like Chaetoceros, Rhodomonas, Pavlova, or Isochrysis (all known "finicky" microalgal species), then you might be better off starting a culture off with Guillard's and experiment with MG as you have the time.
If you do decide to use miracle grow, here's an anecdote from 14 year old me about overdosing: "maybe dosing until the water had a blue tint was a bad idea..."
