Direct answer: Both are fail-secure slide-gate openers that use a crank during a power outage. The Eagle I is the smaller residential model at 18 feet / 300 pounds. The Eagle 1000 FSC is rated to 27 feet / 600 pounds.
If the gate is within the Eagle I limits, the smaller model may be enough. If the gate is longer or heavier, the 1000 FSC has more published capacity.