Thanks to everyone who's helped me out on this one.
For all the beginners (like me), this is what I found:
I consulted a very learned Astro Photographer about this question and he has actually tried and compared both (add + average) stacking methods. The resulting images contains the same information but require different adjustments in Photo Shop.
The moral of the story is, it doesn't matter if you sum or average, the resulting image contains the same information.
I hope this helps someone else to get their head around this question.
For a detailed explanation click on this link:
http://keithwiley.com/astroPhotograp...Stacking.shtml
Erwin