Rat swarm is considerably milder (though comparing X skirmishers to X soldiers isn't entirely fair).
Five 1st level KotFS characters vs 4 Rat Swarms:
Resources spent during combat: 2 Action Points, 1 Lay on Hands, 5 Healing surges.
After Combat: 8 healing surges between 3 characters.
Five 2nd level KotFS characters vs 5 Rat Swarms:
Resources spent during combat: 2 Action Points, 1 Lay on Hands, 4 Healing surges.
After Combat: 5 healing surges between 3 characters.
Five 3rd level KotFS characters (w/ +1 weapons/implements/armor) vs 6 Rat Swarms:
Resources spent during combat: 2 Action Points, 1 Lay on Hands, 4 Healing surges.
After Combat: 4 healing surges between 3 characters.
The rat swarms are fairly well balanced. They require an appropriate amount of resources to deal with. They can still be pretty dangerous, as at one point, I had the dwarf taking 12 points of ongoing damage, good thing he's a fast healing HP junkie. The slow speed of the swarm just about guaranteed that the wizard could dump a double scorching burst on them with the crazy warlord bonuses on the second attack. Combine this with a Divine Strength Breath attack during an action point from the Dragonborn Paladin, and stuf goes down fast. A party without the AoE options may be a bit more challenged, but using a daily power or two should get them through the encounter.