Run, don't walk, to your nearest optometrist and get your eyes checked.

Ubuntu and OS X are not even remotely comparable from a typography standpoint. Using Freetype (the Linux/X11 lib for font rendering), tracking and stem thickness is inconsistent, ascenders and descenders get cut off, etc.

The Freetype team puts forth a valiant effort, but until they're allowed to use Adobe/Apple/etc.'s patented hinting algorithms, there's no comparison.