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 such as Improved Fortification, Zee Jay's Passive Enhancements, and Booster Bloon.
The cheapness of Double Red makes it a fair candidate for Improved Fortification in the early-game.
With Zee Jay's Passive Enhancement, a Double Red does 120 damage if left undamaged.