I would go with Boba and Biggs.
And it's not only because of arena. Meta changes. However they are rebel and scoundrel with ability block and high speed. Whatever meta we'll have, it will be always based on speed.
Boba is tanky, fast and blocks abilities. He's hard to gear, but as we already established, you don't have to fully gear him to get the best out of him. QGJ can be farmed after him and still you can get best results. QGJ is better end game characted imo.
Biggs si a rebel and goes great with other rebels synergies. And his rush can be useful in GW agaisnt empire units.
Both will help you out to get a head start in tournaments and events. Like I wrote, I already missed a few of them, not having enough scoundrels or rebels. Thats why I don't rock any palpatine :(
And lastly here, wrote about it already, getting higher rankings and ammasing crystals early on will help you invest more and stay ahead.
My previous plan was to go for IG-88, because he is a scoundrel and good for P2 AAT.
They are in different places so you can farm qgj and ig-88 separatelly.
Another possibility seems to be Leia for P4?
If you aim for better raid ranks then robably leia is a better choice. But I would choose qgj. More versatile and can be plugged in many teams. His zeta is quite good as well.