Cassandra Cain is THE badass, all other badasses strive to be as badass as Cassandra Cain, Cassandra Cain is so badass that whenever she speaks, Tim Drake gets kicked in the nuts, whenever she speaks, Batman gets laid, whenever she speaks, Stephanie Brown shuts the Hell up, whenever she speaks Jason Todd rethinks his life choices, Cassandra Cain is badass incarnate
Okay, stop pissing your pants with this. BOTH are trained assassins, and both have proven to be VERY lethal, BUT, Cassandra WOULD have an advantage with her body reading. Unless X-23 can recover from throat slitting, neck snapping, or anything else fatal, I give it to Cass.
Keep in mind ALL martial artists learn to react to their opponent's body language (I do this fencing longsword. If you have to stop and THINK about what you're opponent is doing and how to react to it, you're dead). Even allowing that Cass's is trained to a level above that of the average martial artist, it's not like it's a skill Laura doesn't possess at all. And even if Cass can read what Laura's initial move is, this is still assuming that Laura doesn't, y'know, throw a FEINT. In martial arts feints are still executed as real attacks (because if your opponent DOESN'T respond you've just wasted a free attack). This means Cass has to read EVERYTHING, and because she's not precog she has to do it on the fly and respond to each individual movement, along with the associated delays in processing what she's seeing and her own reaction time.
Which is, y'know, exactly what ALL martial artists do.
Ultimately, Cass's "body reading" is probably a comic writer thinking he's doing something smart and creative, without having done any research into ACTUAL martial arts at all, and rightfully coming across as ridiculous to anyone who ACTUALLY knows about it (I have this feeling all the time when I look at DnD's combat rules...)
I'd really like to see this as a Death Battle. I think Cass has more of a chance than she's given credit. I mean come on, a trained assassin who was also Batman's sidekick. That alone makes it worth the watch, but she has superhuman qualifies as well, so it would be interesting if not sexy >.>