azw3 vs mobi vs prc/pdb on a Kindle

Is there a reason to go with an azw3 file over a mobi file on a K3W? On a KT2?

I am asking as I have been using Sigil with KindleImport and KindleGen to update some of my eBooks, mostly adding proper covers to those without them, and KindleGen creates both files. The azw3 versions are larger than the mobi files, often 2-3x larger, so what does an azw3 offer me on my readers that a mobi does not?

I am including prc/pdb in my title as well as I have some of these files as well and might want to update them and was wondering what advantages the other formats would have there. I guess that might depend if the internals of the prc are TEXtREAd, zTXTGPlm or BOOKMOBI formatted.

Dave