The reason I feel full tank is best because of Warwick's ability to take in more damage with his e and since his q, w, and r allow him to get really close to the enemy he is going to need a way to stay alive in the front line. But don't let my build sway you, go a mixture of both tank and damage or attack speed if you are ahead but now Warwick is very mobile and that plus his new found tanky'ness can really help your teammates. Also having more MR will not increase healing.