It is unquestionably less likely that a Mac will be infected by any sort of virus. There are two reasons which have nothing to do with the baseline security of the OS:
Reason 1: Low interest
The first, and most commonly cited, is the lack of interest on the part of attackers. Mac malware is rare because its market share is small. Malware authors want high distribution, and since the Windows share is many times larger than all competitors put together, the target decision is pretty clear.
Reason 2: Herd Immunity
But the second factor is easily as important, though it stems from the same root. Many types of malware (and in particular the type that this company would worry about in this context) use infected machines to attack other machines, targeted at random. This creates a network effect such that more potential hosts there are for an infection, the easier it is for the infection to spread. Unless the vector can effectively target both Windows and the alternative OS, the large population of immune Windows computers would block transmission, conferring a sort of herd immunity protecting the smaller OS.
Since OSX represents such a tiny percentage of the total installation base, not only are there fewer possible targets to choose from, but there are fewer possible sources of infection. A single infected host may never find another to which to transmit its malware, so the threat can't spread.
Combining these together means that the proability of infection on OSX is vanishingly small, well within the noise. If you switch to an even less popular OS, such as Linux, the effects compound further, making your chance of infection effectively nil.
But note that OSX and Linux are not soft targets, as was otherwise suggested.
While these Linux and BSD (the base OS of OSX) are not as popular on the desktop, they're significantly more popular in server and high-security environments. Your "unpenetrable firewall" -- yep, it runs Linux.
So while these are rarely attacked in the context of user-interactive environments, Linux and BSD are constantly under attack in a automated/headless context. These OSes are battle-hardened and constantly vetted by the best eyes the industry has to offer, with many businesses sinking millions or more into their security. And since in this context (as opposed to desktop usage), security matters more than convenience, features, and ease-of-use, this means that when a security-related trade-off is presented, the builders of these OSes more frequently choose security at the expense of other usability points.
A good example of such a trade-off is Microsoft's insistence on doing font rendering in the kernel. The reasoning is that it makes display updates faster, a fact which technically remains true today, but which was much more important back in the '90s when the decision was initially made. As a result, Windows has been subject to numerous privilege escalation vulnerabilities, the latest of which was patched only weeks ago. Windows still does font rendering in the kernel, and will almost certainly see another related vulnerability again in the future as has happened so many times already.
So are you safer with OSX or Linux?
Yes. You could argue about why you're safer; whether it's market share, design, target preference, or any other aspect. But the fact that you're safer is a given. You're less likely to be attacked, you're less likely to be infected. And even the most successful malware would have trouble tearing through the OSX or Linux desktop community because targets are few and far between.