So I currently have 26 points open in my WIP Scum List and I'm trying to decide what build to use for Quinn Jast:

Quin w/ Deadeye, Guidance Chips, Heavy Scyk (Missile), Concussion Missiles for 25 points

or

Quinn w/ Deadeye, Guidance Chips, Heavy Scyk (Missile), Homing Missiles for 26 points

Which do you think might work better? Is there a better torpedo/missile? A better EPT?

Homings are definitely better than concussions - denying the evade, and potentially saving the focus for defense if you don't need it, is a big deal. Deadeye is probably the way to go at PS 6; if PS 9 isn't common in your meta than VI could be another option.

I like the following:

Quinn Jast (18)
Trick Shot (0)
XX-23 S-Thread Tracers (1)
Guidance Chips (0)
"Heavy Scyk" Interceptor (Missile) (2)

Total: 21

If you have the extra point, put VI instead of Trick Shot.

She's a cheap, easy helper. Action economy is important, and she can contribute several times.