Preference I guess.. I never trained my grunts past level 2 so always went with hunters, feels like they die less though because of their range.
The training time is actually shorter for hunters if you're filling up your camp completely
just used your same method, but on D3:S1 , used the same troops but mine are only level four. All I needed is 1 of the healing spells. I put my hero on the griffon side so he could solo those, then I put my troops on the opposite side. Won it and got 1463 xp i think it was. That's almost a level for me each 5 runs. Oh got 7 shards too =D