the build you are on .321 is the latest build for the public, which is the most stable and bug free version. the build .351 is of the windows insiders release preview ring which is released to those who have opted to test beta version of the builds which will later be released to the public. Do keep in mind that these builds may or may not be the final one released to public and may carry new bugs with them when in testing stages or with the insiders.
Microsoft is set to release a new cumulative build / monthly bug fix and security update on 8th November for windows 10 and mobile.