“Pro” is the original version and only available on Android and uses bitmaps (pictures!) for the map images (the “traditional” way) so will need lots of space for offline map storage.
“XE” is a new version which can be used on Android and iOS and uses vector maps (smaller downloads, handles zooming in/out better). XE is still work in progress and does not yet include much of the functionality of the Pro version.
I can’t answer the question about long term use of OS maps but I can’t imagine any previously downloaded bitmaps will suddenly “disappear” if Bing or OS pull the plug.
Pricing of the Pro version is all over the place because there’s tons of US specific you can buy as add-ons.
I think I paid around $10 CAD about 6 years ago and haven’t needed any of the (paid) add-ons…