In short, a master keyboard is the keys of a piano, and a sound module - or a KORG N264 - is the strings of the piano. Sound modules haven't got keys and controllers, the masterkeyboard has. Regarding the QS8:
I've tried a QS7 and got very disappointed. This machine - which is identical with the QS8 except for number of keys - is too much stuff in a too small thing. But it's cheap.