I don't have any experience on Mac but from what I've read about MacOS it is great in functionality!
Sahir would know better, but from monitoring the security lists for some time my impression is that OS X, like any general-use OS, has it's fair share of security flaws too. However it's secure design (BSD heritance) renders most of them non-critical, unlike windows where the simplest vulnerability brings the absolute hell!
Appart from that it would be unfair to judge one unix-based OS as more secure than an other, because there are many factors to consider, most of which are common and unpredictable (like third party software and user configuration). It is more easy to talk about "out of the box" and "default" security levels, since only these are really measurable, but it's only a part of the picture and rarely the only reason of a security comprisement.
About the difference in the WiFi performance that you noticed, it must be related to either the equipment, the driver, or the software used. However the same equipment is available for both PC and Mac with excellent drivers and there is plenty of software that utilizes the drivers to the maximum for every platform, so most likely Apple is only to be commended for picking up and putting them together