"suffocatingly narrow" or "narrow to the point of suffocating"?

The second sentence seems awkward; it tends to introduce the idea of "suffocating for freedom", as in "suffocating for air" (ref.); so its impact on the mind is not a neat one.

The first sentence is better, acceptable. However, the relation of the narrowness to the freedom of her spirit as one of restriction by the narrowness is not given a very explicit implementation through the preposition "for", nor "to", I think. In fact, this is the rather trite, if convenient, relation we find in such statements as "For me, this is not right.", in which "me" is apposed as a standard against which the right is to be evaluated. For these reasons I'd prefer, personally something as follows.

• The confines of the system were suffocatingly narrow, obtrusive on the freedom of her spirit.

I doubt whether there's any reliable objective measure to judge which locution is more common. Regardless, the safe thing to say here is that both sentences are acceptable.

The first may be preferable to some readers because of its greater clarity. The second sentence may be interpreted incorrectly if taken to mean: The confines are suffocating for (with the aim of establishing) the freedom of her spirit. Of course this isn't what's meant, but syntactically the gloss is available. Some quick readers may have to backtrack and reread the sentence in order to understand the idea correctly.

As for prepositions, we may use: to, for, or against.

My preferred rendering:

The confines of the system were suffocatingly narrow against the freedom of her spirit.