First things first:
Out of the Silent Planet is much better than
The Chronicles of Narnia. In fact, now that I look back, I'm a bit surprised that they're writen by the same author.
Narnia seems heavy-handed and altogether too religious by comparison. (Don't get me wrong; I like religion, bu I don't like to be bashed on the head with a poorly realized God figure/lion.)
It's not that
Out of the Silent Planet doesn't deal with many of the same issues; it's that they're explored in more mature fashion.
Narnia is primarily a kids' series, and it shows—the writing is simplistic and the characters often lack multidimensionality.
Out of the Silent Planet gives us more description, a more philosophical plot and some fairly deep ideas at work. While the Pevensies talk to English-speaking beavers, Ransom learns an entirely new alien language. This linguistic bent seems to mesh well with the story's maturity. (Interestingly, Lewis wrote
Narnia after
The Space Trilogy.)
I'm planning to expand this review, so stay tuned for more.