I've been having the same dilemma. I do have a few facts:
You can use a starter formula up to age of 12 months. After that you should try and wean them onto follow-on or cow's milk. The main difference is that the follow-on/progress formula will have MORE iron. Babies are born with enough iron for approx. 6 months, hence don't need as much in a starter (1) formula. Once they hit 6 months, you need to provide more iron in their diet. Some nurses think that u should try to give extra iron-rich foods if still using a starter formula after the age of 6 months. Also u can get the progress formula's with the added nucleotides and amino acids, which are known to enhance brain development, but whether adding them to the milk makes much difference...
I have this dilemma b/c due being an allergy prone family, our immunologist has said to use NAN HA (1) which is the only hypoallergenic (HA) formula that i can find on the market, but it only comes in a starter formula. And I was looking to move grace onto a progress formula due to the 'iron' reason. I started using a progress formula, but when i told the dr he didn't look happy and said the starter was fine til 12 months and can help prevent allergies forming so ... who am i to argue!
i would say keep using the one you and child are happy with, and if you are providing a good balanced diet with red meat (or iron-alternative) then your baby will be fine.
girl born jan 2003