Go with 8S! I'm loving my Savage Flux on 8S. I just recently tried the Zippy 4S 4000mAH 25C packs. They fit in the Flux boxes and have been working great. All for under $200 bucks for both packs. The 1521 stays nice and cool. BTW, the Cen diffs have been holding up great, as has the Flux tranny.