Mobi or AZW3?

I've received a brand spanking new Kindle Paperwhite for Christmas.

I noticed that my recent ebook purchases have all been in AWZ3 format, so...

1. Should I convert all of my old MOBIs to AZW3?

2. Is one format better than the other?

Specifically on the Paperwhite?

Specifically on the K3 (Kindle Keyboard)?

Advice greatly appreciated.

Thank you. :)