I've always used powder and never had problems. My ped has never recommended ready to use over powdered.
I ended up using Members Mark Formula (from Sams) with my last and LOVED it. They have one that is the same as Good Start with Cultures now. I used the regular milk based that is identical in nutrition to Enfamil. It saved me an unbelievable amount of money and my youngest had less reflux and sleep problems then my older two.