On the Tunings page 2 of our site, we discuss the implications of "tuning at the resonance". I'll just say here that it means some Sopranos do not handle the c' note well.
Sopranos had Machete tuning to begin with, a one-line octave open G tuning. From there it went to Rajao tuning, minus the 5th string, which is the modern key of C Ukulele reentrant tuning. The Rajao, however, was a good bit bigger than the Soprano, and so the tuning fluctuated between C, D, and E flat.
May Singhi Breen, the premier Ukulele artist and arranger at the turn of the 20th century, together with the big publishing companies stateside eventually decided that "Ukulele Standard Tuning" should be the key of D. You'd have to admit from an acoustic standpoint, it was the best choice, as any Soprano should sound good tuned to that range of notes.
To get back to C tuning, however, there are a lot of Sopranos, the large majority in fact, that handle that c' note without any problems. If yours sounds good both ways, then here's what I'd think about.
A D tuning will be lighter, brighter, livlier, respond better and project more. This was the sound that made the Ukulele reputation. In the days before microphones, (think vaudeville), these were the most important qualities, especially for a performer.
On the other hand, a C tuning will be mellower and softer, with more of a bass. Of course these are relative qualities - remember the Soprano is a light, bright instrument by nature. If you are playing primarily for yourself, or with a microphone for Youtube, for example, the softer sound may be what will work best for you.