If you're planning on getting multiple/all DLC pieces for TPS, then the Season Pass isn't a bad choice to save some cash, but if you're only interested in one or two specific pieces of DLC, then you most likely don't need to spend extra on this.

Mission-wise, the only non-arena missions you get are from UVH Upgrade Pack 2, so if you're just looking for more missions to play, then buying just that would suffice.