Customizing an existing one is better insofar as: 1. Its usually faster, since you're just writing a plugin or overlay or whatever on someone else's work. 2.

Customizing an existing one is better insofar as: 1. Its usually faster, since you're just writing a plugin or overlay or whatever on someone else's work. 2. Makes it easier for customer to hire other people to maintain it when you've moved on.