I've never used a 6011 but I would say 98% of the welding I've done is with a 7018. It's the easiest rod I've ever used. I had to use 6010 during college for an assignment and it was a PITA.
7018 takes longer to burn. I build boat docks for a living we only use 7018s for huge I beams or when someones tractor bucket cracks. There is no sense in using a 7018 for little stuff like this. Im not going to argue anymore the guy got his truck done and it looks great. I was just giving a tip to him maybe to help him out next time.