Dual boot vs VM

If you use Windows more often, then you really should run it in Bootcamp mode.  If you run Mac more then you can run Windows in a VM.  It really depends on your use case and needs.

For a Mac, you really should get 16 GB or more if you do run a VM, as 8 GB is really not enough for OS X if you have a VM running and taking up 2-3 GB of your RAM.  If you have 16 GB, I still wouldn't recommend allocating more than 6-7 GB of RAM to your VM.  OS X require more RAM than windows, since it has more 64 bit software.  Windows still runs a lot of 32 bit software.  Depending on what you're using in your VM, a dual core CPU may be sufficient.  You'll just split off one core for the VM.

If you run Windows more than 50% of the time, you may want to get Parallels or VMWare Fusion to run from the BootCamp partition so that you can Boot into the faster Bootcamp mode and use the full RAM and CPU when you need it or Run it in a VM.  It uses the same installed OS, but you can run it either as a VM or as a full Bootcamp.  The VM will always be slower, and it splits your RAM and CPU resources.  If you need speed and more resources then use Bootcamp.  If you need your Mac side more, then use a VM from a Bootcamp.

Of the 3 VM software, Parallels is the most Mac like and works to look most like a Mac.  VMWare has clunkier controls, but is a bit cheaper and also runs quite well.  Virtual Box is free, but it's the slowest of the 3 that I've used on a Mac.  If you don't need Windows that much, you can try Virtual Box first.  Parallels seems the best in terms of interface, speed and conformity to Mac.  VMWare runs well if you don't care about how "pretty" the controls are and is $20 cheaper.  If you don't use Windows so much, VirtualBox is sufficient.

Depending on what you need Windows for, you may want to install WINE, and just run your applications that way.  The advantage of Wine, or CrossOver, if you need a nice GUI launcher, is that it uses fewer resources than a full VM.  When I first started being forced to use Mac, I ran Parallels on a Bootcamp Partition.  I eventually got several applications to work correctly with WINE and stopped using Bootcamp, and eventually stopped using Parallels, but I've since switched companies and I really have to run a VM to launch the current software I need to use.  The software unfortunately doesn't run on Mac or with WINE yet.  I'm using VMWare Fusion, but I only allocate 2GB of RAM to it, since I have WINE to run other Windows Software.

I'm actually a Software/Web Developer, used Windows all my life, but now I see a demand for MAC.  At first, I know i'll use the MAC  only 10% for testing .

Then bootcamp it.  You'll get much better performance.  If you have an older system with actual spinning disks, you should just upgrade it to SSD.  It's not that much of a hassle rebooting if you have SSDs now.  They're pretty quick to start up, especially the newer ones.

You can also legally run OS X in a VM on your Mac Bootcamp partition, since it's licensed for Apple hardware.  You'll have to create the empty VM container from OS X, then copy that to your Bootcamp partition.  Next create an OS X bootable installer DVD or USB stick.  Then, boot into Bootcamp, start a VM with the prepared VM image to install OS X.  You can probably just copy the installer ISO to your Bootcamp partition also.  I actually use my Mac side more, so I haven't had to do this.

Are you saying that with Parallels I can create the VM for Windows, then I can either run it as VM or as Bootcamp, depending on what I need to do.

As well, if I allocate a certain memory and hard drive to a VM, can I go back and change it?