Depends on what you're looking for. In The Light is a nice neutral palett. (I don't have it but would buy it if I didn't have too many palettes in general and lots of neutrals.) many of the shadows have shimmer in them. It has Kitten in it, if you want to try one of their most (if not the most) popular colors.
In The Know is totally matte colors, and has some more reddish tones in it.
The only duplicate in the two palettes is ebony, which is black or very dark grey.
I have and like In The Know, but In The Light is probably what I would reach for more if I had both.
i have the in the know palette and really like it a lot, and have been debating getting in the light. if i didn't have another neutral eyeshadow palette i would get in the light in a second ( i really wish i had gotten this one istead of the one i have).
if you don;t have any other neutral eye shadow palettes I would recommend the in the light palette over in the know because it is more versatile and i think you could create a large variety of looks with it. Plus the shade Kitten is really gorgeous and a staple in my everyday look. I have a single of this shadow and will defintely repurchase it.