I think HV is better... but it depends.
If you're just racing, or bashing at reasonable speeds, there really is no reason to go any higher than 6s. Even at a "low" 75A, that's almost 2.25HP! Unless your truck is especially heavy (~12lbs+), I can't see much advantage. Sure, you can use lesser batteries since current consumption is lower, but you still need quite a stack of them. Also, simply mounting them can become an isse with 8s+. Finding a charger/balancer setup that will do over 6s gets expensive too. Chargers/balancers that can do 5s-6s are very common.
However, if you need as much power as you can possibly get, then HV is a no-brainer. How else can you get 5HP+ while keeping currents reasonable? To me this would only apply for the heavier trucks or for the high speed freaks.
To me, any setup that will take over 100A sustained bursts should go higher in voltage.