I wondered this with my first daughter so let me share a little of what I have found out.
All formulas are regulated by the FDA. They all are pretty much the same. The differences are minor and all have the daily requirements for babies. I don't remember which of the many medical professionals I have been to told me that but I've heard it in more than one place. Also, I have sat in the grocery store, Walmart, Target and Sam's Club and compared the labels on each type of formula. They are pretty much the same.
That being said, different children respond to different formulas in different ways. My daughter and my son both had a little more problems with the Similac formula. It made their stools a little more difficult to get out. The Carnation Good Start made their stools a bit softer than the other formulas. I believe it is because their proteins are already broken down and a little easier to digest. Between my two kids, they have at one time or another tried the Similac, Enfamil, Carnation Good Start, Target Store Brant, Walmart Store Brand and the Sam's Club Store Brand.
Since all of the formulas are going to have what your baby needs, here are some things to keep in mind when switching. Babies can have allergies, too. So when you switch to a new brand, be consistent. Don't switch between several brands at once. If you are on Similac and want to try Good Start for example... Wait until you are almost done with the Similac (have 2-3 bottles worth left), then switch to the new brand. This way if your baby has an allergic reaction to the new brand, you still have a little time to go get the other type of formula, because you have 2-3 bottles left. If there is no reaction to the new formula, then go ahead and use the Similac that is left and continue on with the new formula. Don't try more than one formula in a week's time so that you will have time to gage how your baby is reacting to the new formula. (For instance, the changes in stools).
While we tried all of those different brands listed above, we stuck with the Sam's Club brand. We were able to get a larger can of their brand for about 10-15 less than the Similac or Enfamil. We stuck with the powdered formula as well. Using the powdered was more cost effective as well and the formula lasted longer.
You can use more than one type of formula but before you start to do so, try each for at least a week to gage the baby's reaction. This way if she is having a problem on one of the types you are planning on using, you can discontinue use of that brand.
Good luck. I hope my experiences help you out.