Should I just add a KDE shell to Ubuntu, or switch to Mint? Is resource use similar?
Which Mint distro should I use... are they all Debian-based? I assume Debian would be the easiest transition for an Ubuntu user, as well as the most supported for applications?
To answer these two questions specifically, Mint has two bases for its distributions. The main releases are based on an accompanying Ubuntu release and are therefore functionally and resource-wise identical to the Ubuntu version. Mint adds enhancements that are specific to Mint via their own repo added to the Ubuntu source list, such as their Update Manager, Software Center, MDM login manager, etc, as well as their unique desktop offerings. The main releases are the logical move for an Ubuntu user.
E.g., Mint 13 is based on Ubuntu 12.04, shares its repositories and underlying infrastructure.
The other side of the Mint camp, LMDE, is based directly on Debian's testing branch. Mint makes snapshots of the Debian testing repos, debugs them and releases them as Update Packs for LMDE. UP5 was just released last week. This distro lacks the modifications Ubuntu makes, however, Mint tries to make LMDE as equivalent as possible to the Ubuntu-based releases.
Hope this helps explain the Mint world to you a bit