Well that's the fun part about dual pistols. You can decide between knockback, dot-damage, slow or -dmg depending on your choice of ammo. Sadly I haven't found a fitting concept yet.
I tend to like secondaries that do things in addition to damage. Both /Dark and /Rad provide debuffing as well as damage. In my opinion, a /Rad defeder makes enemies melt faster than a /Fire defender because of the debuffing. YMMV.