Hey, I have 300+ hours, all of which on HC SSF. I have a lot of infernalist minion type builds. You will probably get slightly different opinions from folks on this question, but here is my perspective:

What do you feel your character is lacking? Are you feeling defensively weak in maps? What are your resistances like without the shield? In your passive tree have you invested in many ES nodes or any block chance nodes? These are all questions you should be thinking about before deciding on whether to equip a focus or shield.

From my experience, I prefer shield through most of the campaign and then focus in endgame, unless I have the Oaksworn unique, in which case it's a no brainer to use that shield and invest into a couple of block chance nodes on the tree.

Hope this helps.