They seem to serve the same exact function, with some differences, so, which would you use, and which do you believe is better?
But lets see the differences that they have:
Pip-Boy 2000: Has animation's that play when Idle, or on holidays (Christmas.), has an auto-map that detects movement (Wildlife, Ghouls and Humans, Robots, Super Mutants), and can record audio and visual feed (Essentially a built in camera), and on a side note; Has 'Clue' feature, like a notepad but the button is damaged and the feature is unusable.
Pip-Boy 3000: Has a built in radio, built in Flashlight, can store and play audio messages (does not have visual capability.)