Coming from solo'ing a couple jobs to 75, I would go for damage negation vs increased damage. The goal is to get no one to die and the bard has a better shot at it. The rdm might be able to finish off the mob if the nin dies, but the nin stands a higher chance at survival with a brd for most mob types.
I really do not understand the logic behind this. So the mob is slowed a little more, but with a RDM you get both haste and reduced mob acc, and much, much more, with BRD you have only two buffs, two overrated buffs at that.
To explain my logic: If the mob is even slower that means there's more time between attack rounds for the nin to recast shadows. It also means there will be less attacks in the time that the recast counts down. This lets the shadows stay up longer. Couple that with the evasion that a BRD can give and you have a solid duo going. More on evasion in a second...
The BRD can cast resist element spells and songs where the RDM cannot without going /whm. The Bar spells they do have are single target for themselves. You might be thinking LOL-Bar spells. More times than not, they have helped me out of bad situations.
The big question for me at the moment is: Can the RDM give more evasion than the BRD?(RDM Blind vs NIN Blind + Mambo x2 + song gear) The BRD has two huge evasion songs, and things like melody earrings and the like help the ninja achieve a huge evasion build. With the ninja having Blind, the mob acc and evasion + points go to BRD over RDM.
This all goes back to the basic premise of killing things: "Get mob health to 0 before ours goes to 0." If the mob can't hit you fast and you evade often, you don't get to 0 as fast.
And what is wrong with a RDM/NIN tanking some??
My answer and reason behind not wanting a support to take aggro is simple. I don't want my support that's making this fight so much easier to be the center of attention and possibly die leaving me out to dry. While it's not likely to happen with a RDM because of recast ect... it's still a distinct possibility more so than with a brd/whm who doesn't melee as hard as a RDM. Blind itself is a nice chunk of enmity. The BRD shouldn't pull hate off the nin/dnc especially if the ninja is using debuffs(blind, paralyze, slow, defense down and evasion down), heals, DD, and faux voke.
Both RDM and BRD are great selections to assist a ninja. I wouldn't look beyond those two jobs for a duo partner honestly. It is just my personal preference and play style that leads me to brd over rdm. I hope that explains where I'm coming from in a duo partner choice.