The early battles of The Third Interstellar War revealed the short-comings of the multi-purpose Dauntless. It's anti-ship weaponry were too weak and it's anti-fighter to little to have any contribution in war-time fleet ops. Thus the Defiant was rushed into service to support the Big Guns.
After building the Rattlesnake, I felt 2 things. Like after every micro MOC, I wanted a minifig version. And second I felt the Snake needed a support fleet around it. Being impatient I decided to return to a old MOC. But I wanted to see if I could improve the design. Though nothing radical, I think the Defiant just looks sleeker and cooler.