I found that Long Haul was better equipped to handle the destruction missions, especially where you had to destroy buildings. Use his flamethrower and it kills them very fast. You just have to watch overheating on the grenade launcher, plus his heal ability will help in the longer missions where you have to deal with a ton of autobots.
The racing missions, Sideways without question.
As stated, if you have trouble with a mission using LongHaul, give it a shot with Megatron or Starscream and find who works better for you. I just found Grindor's turrets no where near as useful as IronHides, normally they were destroyed 5 seconds after I placed them.
As stated also, knowing where the time stops are is critical on some missions.