Theoretically rk3318 is a cheap version of rk3328. We don't know yet perfectly what rockchip means with "cheap version", what is stripped down and what has been removed from rk3328 that makes it less expensive because there is no datasheet, no official launch and no technical documents about.
It's just my guessing that maybe they reduced the video encoding/decoding media features, but some other people said that the rk3318 has a bit more powerful GPU than rk3328, and on par with s905x (which is a bit strange, because rk3318 should be the cheaper version...)
edit: ah, rk3328 has support from armbian and mainline kernel, 3318 in theory is exactly the same but still does not appear officially anywhere
