I recall that during 10240 to 10586 update process, Windows downloaded the huge update and proceeded to install it as if it was an OS upgrade... There's another PC with Windows 10 10240 that I need to update and I wonder whether I should download the large update and apply it OR just use official Windows 10 10586 ISO and install it on top 10240?
I preferre to use the iso option, i believe the upgrade by wu doesn't let you choose what to keep (it keeps all by default) and the iso gives a choice. The iso could be modified to your preferences, the wu upgrade not.
Would suggest ISO download and upgrade using it. In case you install 10240 and then run update it would download 3/4gb installation. In future if you re-install it would again follow same steps. keeping 10586 iso would make your future re-install simple saving time and data download.
I did a couple upgrades side by side on same hardware, one via USB installer and the other by MCT. Surprisingly the MCT finished way before the USB install. I couldn't tell you why, but I do know the ISO method checked for updates way longer than the MCT did. This was the only time I've tried it tho, and it may not be the case in every instance.