For the same cost and slightly more base damage (but not health) than a Swarm Green Bloon, Double Red Bloon is more vulnerable to damage but is better with health-boosting effects (that don't increase delay) such as Fortify, Zany Juice, and Zee Jay's Passive Awesomeness.
The cheapness of Double Red makes it an especially good candidate for Zany Juice in the early-game to apply quick pressure.