Originally Posted by
Rantzien
In my opinion, Dagger is a more all-round character. She can deal damage in all elements, as far as I can recall, while she still can heal good.
Eiko on the other hand is an exceptional healer and party supporter. While her summons are useful in certain situations, as Endless Twilight says, they are very limited in other.
If you go with Dagger in your party, my advice would be to include someone else with decent curative/supportive abilities as a backup healer, for example Amarant.
If you go with Eiko in your party, you should only need to be able to revive her in the case that she dies, and she should be able to handle the rest of the healing. Quina with Auto-life to cast on her could be a good complement.
As for the thread's original question, "Who is the best summoner?", I'd say Dagger. She is pretty clearly meant to be primarily a summoner and secondarily a healer, and the other way around for Eiko, as suggested by the order in their respective command lists, their Trance and the difference in amount of summons.