It depends upon your needs. I travelled on the Maiden voyage of this ship in December 2007 I booked am inside cabin. The cabin although not large was adequate for sleeping and changing in the evenings. However, it was no smaller than the standard outside or balcony cabins. However, some people prefer a sea view or better still somewhere private to sit outside in the evening, however, the cruise we went to the Xmas markets in the Baltic and so nobody want to sit outside in those low temperatures. Another consideration is whether you get seasick, if this is the case a mid-ship inside cabin on a low deck will experience less movement. If you just want somwhere to sleep an inside cabin should be fine and spend the money you save on a few luxuries.
Opt for a cabin on the lowest deck as these inside cabins are bigger.
All the cabins are well designed and comfy, don't waste money on others they are more than adequate compared to other cruise ships.