Both can do the job.
The quad core CPU will probably give you better performance overall if you have a heavy workload on the VMs.
I'm just not sure that can an i5 handle windows server 2008?? As i said, i'll be doing some security tests running simultaneously on host machine, so host machine will also need some processing power?
If i am not wrong, i5 has two physical cores and 4 threads (logical Core), and i have to denote one physical core to the VM, would one core will be enough to run Windows Server 2008?