Whoops my bad. Fail reading on my part.
Her USB isn't particularly special, considering the abilities she can use sans Lifesiphon all have reduced cast time (Celerity/Thief/Ninja).
Also her BSB is Instant Cast with reduced cast time Commands. It's a carbon copy of Vaan's, except it reduced ATK instead of MAG. So while it would make Lion a debuff stacking machine with her USB and BSB, it makes the other worthwhile effect of the USB (non-charge 2) redundant.
Ayame's BSB remains one of the few BSB's to survive powercreep, due to how self sufficient it is. Contested only by Squall's BSB2, she can reach soft cap all on her own with it on Mage teams and she also has a great LD if you are so inclined for that.
Can't go wrong with either really. But if you were to take a XI relic, Ayame > Lion.
That seems reasonable, looking at her BSB now it seems like she's just a physical (and better?) version of Edea's BSB which I do have. Out of curiosity why would you recommend getting that over Raines BSB though? People seem to make a big deal out of that one and from what little I've seen it does make it seem like he trivalizes two of the MD's.