Armor-Piercing shells from arty will usually kill all but the heaviest tanks in one shot. But any tanker worth his salt will haul ass the instant a shell lands near him, so unless you hit with the first shot, you might have wasted that spot. On the other hand, HE shells only kill the lightest tanks in one shot, usually requiring two shors for medium tanks. The upside is that, with such a huge blast radius, only the most incompetent arty gunners will miss stationary tanks with HE. However, unless you are an arty master, you likely won't be able to get a kill shot on that tank you just damaged as he retreats with his tail between his legs. You can be happy that you stopped him from raping your team temporarily, and you will likely get a kill assist on him eventually.
Also, mortar shells are all but useless against tanks, requiring at least three direct hits to kill a light tank such as a Stuart.