Funny, I've also heard 7018 called a farmer's rod. I like the 6013, 6011 since they leave a nice flowing, nonporous, weld and are much less sensitive to moisture since I never use all 5 lbs at once. Even using the gasketed plastic cans my 7018 rod is noticably moisture fouled compared to new 7018.
60 and 70 series rod are 60 and 70,000 lbs of tensile strength I recall, and your basic A36 mild steel has a 36000 ksi strength so either rod is way stronger than the base metal. I have never seen a properly made weld break. I like the 6013 because it is so darn easy and consistent since moisture is tolerated.