Arena Store :IG-88 or Asajj or Savage??

Savage seems suitable.

88. Vader, Sid and him can do serious AOE damage behind a tank. As long as he's not dead before he gets off a shot, his life steal and AOE as Sid lead is devastating. Once Vader becomes too weak compared to the rest of your squad, he can still be used with other AOE guys or a droid team or with Poggle. After Lumi/ Phasma, Poggle is arguably the best toon in GW shipments. You already have Poe at 5 stars, so the team is ready. Sid, Poggle, 88, Poe and 86/Dooku/ QGJ/ Vader can be devastating.

Savage and Asajj are OK in the right teams, don't see them contributing anything to your lineup. You can ask the guys suggesting them how both of them will complement your squad.

Thanks guys for the valuable input. Need more advice to decide....

Savage recently got a boost as his punching power was kinda lacking
Has huge staying power, packs an above average punch and is great for mopping up
IG-88 ranks higher (13th, vs SO @ 28th) on my scale of speed-adjusted damage, and marginally higher in Damage Landed v Survival Time vs an avg toon
Asajj is lovely but ranks all the way down in 65th place due to her appalling speed @86 and packing a decidedly average punch (pre-buffs). Debuffing taunts are useful but not 100% reliable so while I have Asajj, I'm not spending any resources there

If you want to play droids, 88. If not Ventress is nice in GW with the self heal and as leader of a B team with Daka for fast stun and a key kill. Savage probably will be much better at 80. Up to you.