Personally, I chose Nashor's tooth because it grants CDR plus AP and mana regen. Those are three stats that sona benefits from. I think it's better to get an item like Nashor's because it gives a mix of all three. (CDR and Mana regen are most important, but having one without the other is pointless. And AP is always a good thing.) Sure there are other items that give better benefits in each category, but few that provide all three.

I'm glad someone stepped forward to revise Sona's build without the locket. However, I'm a bit skeptical about the Nashor's Tooth plus Soul Shroud, especially with how you mention CDR as its best benefit. Last time I checked, CDR maxes out at 40% and Nashor's puts it at 55% and Soul shroud kills it with at 70%. If you're going for Nashor's for the CDR, don't. There are more powerful items that give ability power.