A discussion recently came up on why you usually notate in Db rather than C#. Here is the theory behind the topic.
Excerpt from
www.cyberfret.com/theory"B is the same as Cb, F# is the same as Gb and C# is the same as Db. These are known as enharmonic keys (sound the same, but are spelled differently). As far as practical use, you would use the B instead of the Cb. It is easier to think of 5 sharps rather than 7 flats. Also you would use Db instead of C#, because it is better to think of 5 flats instead of 7 sharps. Since F# and Gb have an equal number of sharps and flats, both are used.
C# and Cb are considered more "theoretical" keys than actual usable keys, so for most situations do not use them."