Keep 2006 330xi w/102k miles vs get a 2010 528xi CPO

I would keep the 2006, the water pump and battery are normal wear items, no big deal. The car should last a long time since you've been taking care of it. Personally I would hold onto the 2006 until you can afford a F30.

F10 vs. E90. Personally, these are two every different yet similar vehicles in my book. The F10 is much roomier. If you have more than a party of four full-sized passengers, then you have your answer. As for maintenance, both share the same drivetrain and underpinnings. I assume that you have driven both vehicles and noticed that the F10 rides very different. It really comes down to personal preference here and how long you like holding onto and maintaining your vehicles.