firstly... Untill you know its what you want to do, perhaps going for a cheaper webcam like the Phillips SPC900NC ToUcam is a better idea than going all out on a DMK..
The mono DMK will yield better results due to the full resolution of the camera being dedicated to luminance, while filters handle the color, where as a color DMK splits its resolution up in order to capture colors... There is a good article on this
HERE. However the mono camera will require 3 times the post-processing to get the final image, and for a beginner, I would imagine it would be a little overwhelming.
Firewire is better than USB due to its higher transfer rate. This means that the data (image) is not compressed by the camera before being sent to the capturing pc. resulting in a better image
K3CCDTOOLS is a great program for capturing planets/moon..
I have not used the ETX-105 so im not really able to answer questions about it... im sure someone will soon