The hardware is identical on both cards as they have same amount of DSP power and RAM.
You can run the same amount of plug-ins on them.
You can use all optional plug-ins with both.
So essentially the only difference is the bundle of plug-ins you get with the PCI mkII. PowerCore Unplugged is also ideal if you wish to use one only with select 3rd party plug-ins, rather than the included plug-ins. If you already own a PowerCore.PowerCore Unplugged is an great solution for you, as it expands your DSP power by allowing you to run both the included plug-ins and any 3rd party plug-ins you may already have registered (provided of course that you don't leave the PowerCore Unplugged the only PowerCore hardware in your system -- if so, please see the section above for further detail).