pianoguyy made the most important point:
The POD will give you a "recorded" sound, because of the microphone and the cabinet that is simulated. If you like the sound of your CDs through your monitors, than you will also like the sound of the POD. But, metal guitars are doubled, compressed, whatever… so you will never get that Metallica "wall of guitars" with only one guitar track. But you can always double your tracks, this is where the fun starts :-) And also a very good exercise for your timing.
For Metallica sounds I would start with the Threadplate or the Soldano simulations. Try different cabinets, mics, before you start using to much EQ.
I have the HX Stomp with the Mission Engineering Expression Pedal. Nice combo. If you don`t use MIDI maybe the POD Go is the better solution. For playing live I think the POD Go would be better, the HX Stomp looks like an octopus with all the cables…. but, you could also use your POD HD500 to control nearly everything from the HX Stomp per MIDI. Not an easy decision :-)
For me the biggest difference between the POD HD and the Helix is "less tweaking". With POD you have to dig deeper to get a good sound, Helix just has them with less tweaking.