Don't decide to get a Mac or use Linux because they're more secure. THEY AREN'T!!! There are more security issues with windows because there are more windows machines, they're a much bigger target.
Mac OS is a variation on Linux and Linux has as many potential security issues as windows, its just that most people prefer to pwn windows machines.
So how do you stay secure? All depends on how paranoid you want to be!! Basic security is windows with latest patches, firewall and AV. Ultra paranoid is BSD Linux running a web browser in VM and reset the VM snapshot every time you finish surfing.
No it isn't.
Mac OS grew out of the work that Xerox did at PARC in the late '70s and is not unix based. However, you probably mean Mac OS X, OS X was based on NeXTSTEP which used BSD unix with a Mach Kernel and has no common codebase with Linux.
BSD and Linux are two entirely seperate operating systems. Berkeley Software Distribution (BSD) is a genuine Unix that was based on the original AT&T Unix. Linux is, strictly speaking, the kernel, the rest of the OS is provided by GNU and whilst unix-like, by definition GNU's Not Unix.