Personally I magnetized the arms so that I could swap out the gun pod, club or open hand. I found the Spartan arms to be very easy to magnetize and my modeling experience consists entirely of RRT at this point. Barring that I would add the gun pod. The upgrade is cheap and worth taking most of the time you field the Spartan anyway. If you don't take the upgrade it's easy to just say 'these Spartans don't have the gun pod this game".
Wouldn't it also depend on if those you are playing with require WYSIWYG (What You See Is What You Get) or not? I prefer to go off what is on my army list, rather than have to work with so many different mini's or magnets.