This topic is one that confuses both beginners and advanced witches alike. I learned that Magick is more suitable when describing witchcraft because it differentiates between parlor tricks and "real" magic.
Dictionary.com states the following: magic: the art of producing illusions as entertainment by the use of sleight of hand, deceptive devices, etc.; legerdemain; conjuring: to pull a rabbit out of a hat by magic
However, right underneath it also states: the art of producing a desired effect or result through the use of incantation or various other tehcniques that presumably assure human control of supernatural agencies or the forces of nature.
So, we can see that the word "magic" already includes witchcraft. The k was added later and popularized by Aleister Crowley. He believe that magick is "the science and art of causing change to occur in conformity with the Will."
Dictionary.com defines magick as: Archaic. magic. or: a power or effort associated with Wicca.
By this definition many magic practitioners might not want to add the k so that they don't affiliate themselves with Wicca, which is perfectly fine.
Some, like myself, know that many other Witches and Pagans use the k so that tagged material can be found more easily. So, I guess it all comes down to preference. If you don't want to be associated with Crowley, maybe you won't want to use the term he is linked with. Or, maybe you don't want to be linked with Wicca. Both are valid reasons.
Either way, make sure to keep tagging posts so that we can all find each other!