They actually completely overlap 100%.
Google Voice
Using your own current number, you are limited to these things:
- Google voicemail: voicemail like email
- Voicemail transcription: read what your voicemail says
- Custom greetings: vary voicemail greetings by caller
- International calling: low cost calls to the world
- Notifications: read voicemail messages via email or SMS
- Share voicemails: forward, embed, or download voicemails
and if you use your actual google voice number with your phone you get all the above as well as:
- One number: a single phone number that rings all your phones
- Free SMS: send, receive & store text messages online
- Block calls: send unwanted callers straight to voicemail
- Record calls: record phone calls and store them online
- Conference calls: join several people into a single call
- Screen callers: hear who is calling before you pick up
Since I was getting a new phone and like to get a new number every few years to clear out old contacts I don't want to bother me anymore, I jumped at the chance to go full google voice. The way you can completely integrate it on Android 2.0 on the droid makes it insanely easy.