First why oppose having a robot and using a tablet? I think both approaches are actually quite complementary (no I’m not advertising my own phd here ^^) and are both really interesting in the context of the Cherry Project.
Second, I would say that a very important point when using robots and especially humanoid ones is their physical presence. They are embedded. You can look at them, touch them, see what their doing…
This is as you say very important for empathy. As we are social creature we really like to interact with others but also with creatures. It has been shown that interacting with pets could really improve mental health of elders for instance. Yet, their usually not allowed in hospital and robots could be a really interesting substitution in this case. You can look at the very interesting (and successful!) Paro robot. Even really simple interaction - we are far here from human-human type of interaction - can already be really stimulating.
For your specific question, I think it really depend on what type of information are convey by the robot/tablet. I guess you have to be more specify here. For instance, you can find studies trying to compare gaze of humanoid face versus using a screen here. You should find other interesting paper in their related work. I’ve seen (I can’t remember where) studies comparing smileys versus a robotic face mimic emotions.
Two other related very related european projects:
(I’ll complete this post when other references come back to my mind).