Actually, you're both wrong, in my opinion. :-)
Overall market share has some to do with the success of worm propagation,
but the real problem is market share diversity at all levels. IIS is
plagued by worms because one piece of code targeting whatever version of IIS
is widely used can typically infect ~ 95% of the vulnerable portion of the
IIS market. Multi-platform products like Apache, on the other hand, have
the advantage of portability (i.e, variations in the underlying systems
within its market). A fantastic example of this is Scalper -- it targeted
Apache 1.3 running on BSD/IA32. A very small portion of the market for
Apache 1.3.