At this point in time I do think Ellison is a slighty better 13, and will fully deserve to start at 13 if he gets a chance. However I think Ben Smith is potentially a better 13, and indeed has the potential to be a world class 13, as I see so much of Conrad Smith in his play. Indeed when they were playing together in the last AB's test I had trouble telling them apart on several occasions as they have a strikingly similar running style!
I really like the physicality that Ellison brings on both attack and defense, but I don't think his distribution skills are that great. He doesn't really create things for other players - he either has a go himself by cutting back against the traffic (usually very effectively), or simply passes the ball straight out to the wing. He is certainly not the worst distributing centre going around, but the AB's have been spoiled by having the best distributing centre in world rugby for the last 5 or so years, and with world class outside backs I think a strong distribution game is a must for any AB's centre. This is the main reason I think Ben Smith has so much potential in the centre position, as he reads the game brilliantly, has great handling skills, runs with the ball in two hands, and has an outstanding ability to straighten the line with ball in hand and create space for his players outside.
I still think that fullback is Ben Smith's best position (indeed I would start him at 15 for the AB's), and it is certainly the position he is most valuable for the Highlanders/Otago, beacuse as you say he works well with a bit of space to move. He hasn't received that space in the few times he has played in the midfield for the Highlanders/Otago in recent seasons due to being part of some very average backlines - outside a backline of of Smith, Carter, and Nonu I think he would get a lot of space, and would create even more space for his outsides. When he entered the (far more functional) Highlanders backline in the midfield this season he always created a lot of space.
I think a lot of my faith in Smith at centre is from watching him play there at club level for Green Island over the years, where he just looked at a completely different level to any other back in Dunedin club rugby....