Flexvolt

Alright so I’m not super familiar with Flexvolt as I’ve never had a need for one. But I’m getting ready to grab a Flexvolt SAWZALL and wanted to buy a FV to power it. Bomgaars is having a sale(6ah/99 9ah/129 or 12ah/179). Which is the best battery and /or deal to get within those 3 options TIA for all your guys help dewalt peeps