I got what I guess is the final word directly from MasterWriter and that is, basically, NO, you do not get access to both versions with a single license! It may not matter much since both are very similar but I'd really prefer access to both interfaces. Below is what they said,

Originally Posted by MasterWriter
The CW and SW versions have the same tools, the differences are negligible. The only notable differences are that the order of dictionaries is slightly different in each and the info page of the songwriter version contains fields related to a song, such as writer/publisher info, key, time signature, length, bpm, etc. Otherwise, the two programs are identical in terms of content and other. Either can really be used to write anything.

If you write songs and are also a creative writer, I would stick with the songwriter version in case the Info page is helpful in tracking information related to your songs. But yes, I can switch you the creative writer version very easily if needed.