Yes they will update as one
Yes, because they are one and the same they do update as one. Although Parallels does create a folder to store files when using the bootcamp partition in Parallels. It should be accessable in Bootcamp because Parallels installs tools.
I am in the camp who ask why have both? BootCamp is nice if you can focus just on one OS running at a time. Parallels is nice if you prefer Mac OS but need to run a Windows only program(s). My issue with Parallels is you not only have to be wary of compatibility with Windows but also with Parallels. Your adding a third equation to the mix. Most of the time its fine. If you can deal with the bugs at times the Parallels is easier. If you prefer performance and solid reliability with Windows go with BootCamp.