My opinion is keep the Fischer Price starter irons until - at minimum - level 59. This is when the level 59 G25 irons become available. Although I've never used either set, I would personally rate the g25s as a FAR superior set to the Nikes simply because of their trajectory - High for the Pings vs Medium for the Nikes.
Myself, I decided to bypass the G25s altogether (formerly the G20s when I hit level 59) and I waited until I had unlocked the Rocketbladez at level 79. Although they do have a faster meter than the Pings, they also have just a hiccup more spin and a tiny bit more precision. Its just a question of whether or not you want to deal with the faster meter.
When it comes to the Pro Shop my general rule of thumb is unless money is absolutely no hindrance and you have a seemingly unlimited supply of credits, only make a purchase that has long term value. To me, buying the Nikes at 52 and then potentially replacing them at 59 for the G25s just isn't good credit management, but to each his own.