Compare (all these will fit a standard old e-maxx tray) 120 , 100 , 50 , 20 , 10 amps

Sky Lipo 40c softpack 2sx5000
ACQ 40c hardpack 2sx5000
Turnigy 30c Hardpack 2sx5.2

I'd buy the Turnigy or the Sky if I had to fit in old 6 cell trays. If soft pack then get the Sky. If hardpack then get the Turnigy.