- Caller ID
- Voicemail/Answering Machine
The differences are huge, when it comes down to it. One of them lets you be totally neurotic, and log all of the incoming calls you get, no matter how mundane (eg: MCI, Friend #1, Friend #29, etc.), and the other lets you filter the call-backs (if someone didn't have enough to talk about to actually leave a message, what's the point in giving them a call back?).
Fortunately, there is an element of screenage available with either option. I'm an avid (if somewhat ashamed) call-screener. I like to know who I'm talking to before I answer the phone. The answering machine makes you wait out the ringing, which is a minus, but the caller ID is a pay-as-you-go function, and I'm more inclined to a one-time-only fee.
If you had to choose, which one would you pick?