I'll make it quick: I guess it's about personal preference I suppose but yeah, they both are developed with different goals in mind:
If you want a large community, and get involved in RP while still experiencing a "vanilla" experience with a few additions try Discovery.
If you want an expanded universe, filled with new content and new places to explore with an "overhauled" experience get Crossfire.
Thats from my perspective anyway.. Just snoop around and find what you need best.