Yeah, the three swords are better and worse for different levels. I'm not all that impressed with the Life and Summon swords.
Life's healing is too little to make up for the damage done in later levels. It's really only good for the 3rd level with the whale (the name escapes me) until you level it up way too high.
Summon gives you meat shields, which basically makes it Life+ right off the bat when dealing with the enemies that Life is good against. But tough enemies chew through the summons too fast, and it is useless against bosses where you have no enemies to kill for new summons. It's pretty good on the levels with ghosts (Castle Stairs, Hell), but isn't that amazing on the rest of them. Even on the ghost levels, you can still use an Imp scroll for the same effect.
Fire is just a lot of damage. It gets you through levels fast, albeit not necessarily with all your health. Against bosses, it means you don't need to use as many potions, seeds, and scrolls, and that you don't have as much time to mess anything up. Once you get the Unicorn Horn, the healing between enemies makes up for most of the defensive benefits of the other swords. It's the best sword for the Castle Entrance, Castle Keep, and everything beyond Hell.