I can't say which position is the "right" one, but I also noticed different distros have different defaults on where the tabs are positioned.

It's cool that KDE lets you do that, but it's a bit annoying actually as it messes with the consistency of KDE. Sure, users can always change their preference to what suits them best, but it would be nice if out of the box all KDEs behaved and looked the same and leave the personalization to the user after installation.