Mac OS X uses a Mach kernel and FreeBSD utilities at it's core.
No, it is unix-based but Linux is a kernel not an operating system.Ubuntu,Linux Mint,Debian,and puppy Linux,ect. are OS's that use the Linux kernel.
Ubuntu uses the Linux kernel, which is a monolithic kernel with loadable modules.
Based on its sheer configurability, a modern Linux kernel takes the cake by far. it is still possible to use a modern kernel with a basic userland in 4 MB of RAM.
2.6.29.4
These are separate and unrelated tasks. To check the version of the running kernel in Linux, use the command uname -r. To upgrade the kernel, either use your distro's package manager (if any) to update the system, or download and compile the Linux kernel source.
They both support Windows, Linux (& other Unix variants such as BSD), and can also support Mac OS X. However, most Mac OS X use primarily Intel instead of AMD.
Linux is only a Kernel (Operating System). Different Flavours of linux have different user programs on top of the same linux kernel. A high level example : Ubuntu has the user program(package) GNOME while Kubuntu has KDE, whereas both ubuntu & Kubuntu use the same Linux Kernel.
2.4.7
Mac OS X is based on BSD, which is similar to Linux. Because of this, it is fairly easy to port programs between the two operating systems. However, the two operating systems are not the same, and programs for one don't work on the other. Linux also uses the X window system, and Mac doesn't, though Mac has a program to support programs that use the X window system. Because of this, Linux programs can be configured to run on Mac OS X with a little elbow grease, but not the other way around.
Linux is a monolithic kernel. Some operating systems with a microkernel use Linux as a process for providing drivers, but this is irrelevant to mainstream Linux.
A Mac is a computer the cost of which varies depending on which model you buy. Linux is an operating system that is free. You can use the Linux operating system on a Mac if required.
This obviously depends what platform your writing your program for, if it's for Linux/BSD/Solaris etc, use Linux. If it's for Windows guess what you use? Windows (Except in some cases using Linux is okay). If it's for... you get the point. Personally, I use Linux.