So I haven't played in a tournament in a while and I want to run something to counter Poetensity. My main idea is Ion. That should be able to slow down Poe and keep captain nym's at a reasonable distance.

Which build would work better?

Kath Scarlet: Predator, ion cannon, e.u., tail gunner, hot shot blaster

Captain Nym, vi, havoc, genius, autoblaster turret, bomblet, long range scanners, ion pulse missile and ion torpedo.

The torpedo may let me ion a small group and then I can drop a bomlett on them and range 1 autoblast for insult to injury.

Or

Kath Scarlet: Predator, ion cannon, e.u., tail gunner, inertial dampener

Ketsu Onyo: VI, Gyroscopic targeting, k4 security, shadow caster, rigged cargo chute and black market spliced tools.

If Poe thinks he can fly anywhere he wants, he hasn't met Ketsu Onyo. Ideally she tractor beams people right into Kath's rear arc and leaves them defenseless.

What do all think?

I think there are better ways to counter Poetensity like all the good old Scum lists below, which will take him down by a combination of alpha-striking plasmas and Fenn Rau and denying his focus.
I really do not get the big hype about Poetensity. He is hard to kill if you fly him defensivly, but for a 40+ point ship his offence is very weak. Most of the time he will rolling 3 attack dice with only his pilot ability to modify one focus die to a hit - not scary at all. While a ship like bombing Miranda scares me like H***

Fenn Rau, Mindlink, title, AT (32)
Old Teroch, Mindlink, title, AT (30)
Contracted Scout, Mindlink, Plasma, EM, K4, Unhinged, Chips (35)
97 points

OR

Fenn Rau, Mindlink, title, AT (32)
Palob, Mindlink, Cikatro Visago, Cloaking Device, title, Pulsed Ray shield (28)
Contracted Scout, Mindlink, Plasma, EM, K4, Unhinged, Chips (35)
95 points

OR

Fenn Rau, Mindlink, title, AT (32)
Asajj Ventress, Mindlink, Latts Razzi (40)
Contracted Scout, Mindlink (26)
98 points

OR

Contracted Scout, Mindlink, Plasma, EM, R4, Chips (33)
Contracted Scout, Mindlink, Plasma, EM, Intelligens Agent, Chips (32)
Contracted Scout, Mindlink, Protons, EM, R4, Boba Fett, Chips (35)

OR

The Dengar/Tel world-winning list of 2017