In Italian there is a tendency to perfer an impersonal construction such as the infinitive to avoid repeating the same subject conjugation more than once.
So you say
Penso di amarti = I think I love you.
So di essere giovane = I know I'm young
Mi chiedo cosa fare = I wonder what I should do
I don't know if this sentence would sound acceptable to native English speakers but it is perfectly normal in Italian, although as Civis mentioned the meaning is but fuzzy because of "preferire" instead of "scegliere".
I guess it is due to the fact that "preferire" usually means a clear choice, so it is weird to us it this way.
But it would make a lot more sense if you said "Non so quale preferire" which would mean "I don't know which one I'd prefer".