I don't think you are really asking about an "emulator".
There are two ways to run Windows on an Apple computer.
1. Use a Virtual Machine by installing Windows using Parallels, Fusion, or Virtual Box
2. Install Windows natively using Boot Camp Assistant
Regardless of the method selected, you must install a legal version of Windows. Windows is not emulated, it is installed from an installation disk.
If you use a VM then Windows shares your computer's resources with OSX. Both OSX and Windows are running at the same time.
If you install Windows natively using Boot Camp Assistant then Windows is booted independently of OSX and can therefore make use of all your computer's resources.
So for heavy resource use applications using a Boot Camp install will result in better use of your computer's resources. But for ease of transitioning between using OSX and Windows the use of the VM is better.