From the problems posted in the technical forums (eng. and german) the majority of Moodle sites run on Apache - which also reflects the situateion worldwide.
In the thread I've mentioned was this comment too:
http://moodle.org/mod/forum/discuss.php?d=25033#118224
OK, there are others who have no problems with WIMP or WAMP. (BTW replacing the MySQL with MS-SQL seems to be harder.)
My point is, the choice of the plattform is not only technical, you have to take financial and political aspects into account.
Even on purely technical basis, we don't have fair comparisons of Moddle on LAMP vs. WAMP, nor WAMP vs. WIMP.
Financially, MS asks for licence fees whereas LAMP is free. The counter point is that, the "switch over" also costs money in terms of training. The discussion is an involved one -YMMV.
Politically, well that goes off-topic