Encoding.ASCII vs. new ASCIIEncoding()

If I use:

A) var targetEncodingA = Encoding.ASCII;

and

B) var targetEncodingB = new ASCIIEncoding();

then both targetEncoding0 and targetEncoding1 are of the same type.

Are there any preferred scenarios and/or advantages/disadvantages when to use A or B?

(besides creating new instance by constructor each time I use it)