I prefer Chain Bonus Lv.5 weapons (Grasitha and Gandiva).
I feel that while ATB +50% is good, there are fights where I would prefer to switch them out for the above (such as Ispusteke and Gilgamesh). I feel the Chain Bonus weapons are more universally applicable.
The following link provides some further insight: http://etrosgate.com/ffxiii-2/inventory/weapons/ultimate-weapon-analysis
And if you're interested in seeing something only Chain Bonus weapons can do: http://www.youtube.com/watch?v=FiJDawezEk4 --- GT: geblobledee http://www.youtube.com/user/MightyGeb