We use powder for our little one. Pre-prepared formula is quite expensive (here anyway). The powder has worked out just fine. I've also heard on B&B about it not being sterile and the only way to make it sterile is to mix it with water over 70ºc. But, I've read not to mix the powder in water that exceeds the temperature stated on the back of the formula box as this destroys important nutrients. I follow the instructions exactly on the back of the box so I can only speak about the formula we use - it's stated to boil water and wait until it's 50ºc to mix with the powder. To make the process go quicker we use the Cool Twister, cools boiled water down to a desired temperature in 80 seconds, love that thing! So, I'd personally go by the instructions on the back of the formula box if you decide to go that route.