Definitely the 9L on 12 cells. It should work pretty well. You may get lucky with a 12t pinion (with the 9L on 12 cells) depending on the size of the spur (in teeth) and the ratio of the diffs.
SH Z-Car, Custom Crawler, 8s Savage, 12s XTM XLB 1/7 buggy, 4wd 4-link rear/IFS Pro4 truck, Custom Hyper 10 Short Course, Belt-Drive Mammoth ST 1/8 truggy, 4s 17.5 MM Pro HPI Blitz