There are actually three options.
Dual boot with Boot Camp.
VM using Parallels, VMware or VirtualBox.
VM using Parallels or VMware running the Boot Camp Windows installation.
The third gives you the advantages of the first two, i.e. you can make the decision which option to use every day or hour whenever you want. The second option also has three sub-options: you can run Windows fullscreen, in a window, or combined with Mac OS.
Now, generally the first option is ideal when
a) You really want to use Windows, not Mac OS, but thought that Mac hardware is nice and that Mac OS is a good extra.
b) You really want to use Windows sometimes.
c) You want to run games or other programs that want direct hardware access via the OS only.
d) You fear that system resources are not enough to run both Mac OS and Windows at the same time.
The second option is ideal under these circumstances:
a) You really want to use Mac OS all the time and only need Windows sometimes.
b) Or for specific applications occasionally.
c) Or for specific applications all the time (for example Microsoft Access or some such Windows-only application).
For all three running Windows in fullscreen mode will work, for a) and c) running Windows in a window is good too, and for c) running Windows in combined mode (called "Coherence" or some such thing depending on VM product) can be quite good*.
(*I run Powershell a lot because I use it at work and need to study all the time and Powershell is the only Windows program I run all the time. Hence I like the fact that using Parallels it is the only Windows window I see on my desktop.)
The third option is ideal if you really want two computers in one, leading to combinations of all reasons above. I use Windows 7 and Lion with Boot Camp and Parallels myself.
Hope this helps.
Bootcamp allows you to install and run Windows 7 natively on your Mac as if it were just any other PC.
It allows you to co host a Windows installation on the same drive as your Mac OS one, allowing you to dual boot. In addition to being what is effectively a simple boot manager, it includes a full package of drivers for all your integrated peripherals like the trackpad, FaceTime camera, odd Mac keyboard layout etc etc, that makes it easier to use without having to struggle getting things to work.
It does not allow you to virtualise Windows to run within Mac OS, but a handy side affect is that many virtualisation tools such as VMWare Fusion and Parallels desktop will open your bootcamp partition as if it were a virtual machine, allowing you access without a reboot at the cost of performance. Booting directly into windows via bootcamp should be just as fast as on any comparable spec PC
Best Answer
Have you changed your info.plist in boot camp assistant so you can use a .iso file for windows 8.1?
If you havn't follow my steps!
Find Boot Camp Assistant then DUPLICATE IT put it on your desktop right click it show package contents open info.plist then find PreUSBSupportedModels and remove the Pre JUST THAT save and open! then download a ISO
( Look up a activation key )
Here is download ( Dont need key to download )
https://www.microsoft.com/en-au/software-download/windows8
Always STAY cool!
By the way this is just so you dont lose storage on your HD Harddrive