My thoughts. Either will probably work fine. I always find it best to dig in good organic material before you start and be pretty sparingly at first with any fertilizer. It's real easy to over-fertilize.
Miracle-Gro has a pretty good balance of nutrition. I thought it would mostly be nitrogen until I looked.
There are all kinds of different fertilizers you can add. Different flowers, or vegetables for that matter, have their own requirements for different nutrients and soil pH. Try to match whatever fertilizer you use if you have a need to add. For a lot of things, like daffodils naturalized in the yard, I don't add anything. And there are a lot of other nutrients than just nitrogen, potassium, and phosphorus.
Personally, if I had access to it. Rabbit droppings or aged chicken or cow manure would be in the mix for sure. I hope OldGals flowers turn out beautiful!