My company uses a combination. Our subsidiary company, a graphic design/advertising company uses Macs. That's obvious, as most graphic company's use Macs with Photoshop and Illustrator running all day, as well as photography (even though our turn tables only work in Windows, so they use a VM).
My primary company, however, uses a combination of both. Mostly our developers are using Macs, which in my IT viewpoint, its pointless. They are doing nothing but coding, and it can be done on any platform. Instead, my organization gives the developers what they want so they stop complaining. In their eyes, however, the dev's ask for Macs because they aren't as locked down as the Windows systems are in my organization. The supervisor that runs this team is providing his team systems they don't even want. I've talked to several Dev's and they prefer to code on Linux or Windows, not a Mac. However, they aren't given a choice as the supervisor only will provide a Macbook to his team. Again, it comes back to the fact that our organization currently doesn't have the Mac "locked down" as the other machines in organization.
However, that will soon be changing.
A. Yes and No, as we have a combination of both.
B. Developers and our Advertisement/Graphic Design subsidiary company.