hi
we were using the heinz ones with valve.. stay away from those!! expensive and milk gets stuck in the valve and impossible to remove. we have 2, one for water, one for milk. my mchn said use ones without the valve. that seems a lot easier to clean and all.
we started w the sippy cup at 6 mths and he's still using it at 14mths.
personally i dont think you need to use sippy cups or straws at different ages. they can continue using whichever just to stop using a bottle. i think it;s a marketing ploy to buy different types of cups!!