A concert neck will give you more room between the frets than a soprano neck. For some people, that extra space is especially helpful, especially when playing above the 7th fret. If you intend to play on frets 7 and higher, you will appreciate more room. If you have small hands, and have a problem playing chords with long stretches, and normally play on frets 1-7, a soprano size might be more comfortable. Width of neck is more of a manufacturer's choice, with a few brands offering slightly wider necks (Kanile'a). Oscar Schmidt offers the widest neck on one of their models.
–Lori