If you look at the back of the formulas and whats actually in them, you will see some of it is the same - but the amounts of minerals and vitamins are different.

Miss M is on a cheaper one now because I compared the amount of iron that was in them, and got the one with the least amount of iron - now she has no constipation issues.

So I would say the "base" mix of them is basically the same, but the minerals and stuff are in different amounts, which is why one would suit one bub over another one.