The two are going to be roughly equivalent (Mint is derived from Ubuntu, both are using the same Window Manager/Desktop Environment). You may want to give some of the other lightweight options a spin (which you can do without installing a new Linux! You actually get to choose on login!).
Some fine options are:
Fluxbox (my personal fave, be ready to edit config files but it is the ultimate in flexibility)
This is not a complete list, just some of the ones that come to mind