Is Ubuntu+Xubuntu-desktop exactly the same as installing Xubuntu from the ISO?
If not, what are the differences?
Xubuntu has the XFCE desktop, but does not have the Unity desktop, whilst Ubuntu + Xubuntu-desktop would have both.
Ubuntu + Xubuntu-desktop will use more disk space, due to extra packages for Unity etc
The Xubuntu-desktop package should install all the packages that Xubuntu would come installed with - from here:
Ubuntu System Metapackages
xubuntu-desktop: The XFCE desktop environment, and all the software distributed with Xubuntu