For the really young kids I imagine 48 to 72 volts is good. 120 to 144 for the senior kids. I'm no expert on Lithium so we are running lead. Odysseys seem to be the favorite as far as lead.
And the controller choice has alot to do with what dictates your voltage.
Since Jrs bracket race competitively it's more or less what your dial-in is. If you can get your car to run consistently, a programmable controller and some stiff batteries work in your favor.
I'm hoping with the Zilla we can dial in some pretty consistent times. I've personally seen Shawn Lawless and Dennis Berube work their magic at the track with the Zillas dialing them into a thousandth of a second of what they wanted. When I test drove Shawn's Jr, after I arrived back in the pits he grabbed the timeslip out of my hand and yelled in excitement that we got the ET within 1/1000 of what he was hoping for. That was running on 72 volts of Lithium. I forgot the times and ETs though but it was around 76 mph in the 1/8th. I want to do it again!!