The dog companion is widely regarded as the best first-companion choice regardless of your class simply because it does the most damage.  The cleric (healer) companion heals so seldomly and for so little that it really isn't worth it unless you are playing a Guardian Fighter who is already fairly competent with the block so that the small heals she ends up casting are just topping you off since you are always fairly high.

The dog does a decent burst damage attack and has a knock down that he uses somewhat intelligently.

Of course, this is all based on the first 20 levels or so.  Once you max out the cleric companion for instance, she is healing pretty well as she has her HOT as well as her standard heal.  Naturally all of the starter options are going to be worthless once they hit companion level 15 (this will probably take a good 20-30 levels for you because its easy to forget to send them to train).  That is until they add the companion training books that allow you to raise their caps.