The most important thing to understand in this decision is that whatever you choose, it becomes a part of what you will be delivering, almost as if you wrote it yourself. The customer won’t care who wrote what or how well it integrates; they only care about how well the end result is. You can’t tell the customer “It’s not our fault, the gadget library was full of bugs.” From their perspective you screwed it up.
This article has so many great points. Picking correctly now will save you headaches down the road.