I am putting in a shade sail that will be supported by 2 3" 10' posts and 2 3" 8' posts. The posts will be in holes 4 and 5 feet deep respectively.

Question: is there any difference in strength for this application in using that premix concrete you pour in the hole dry and add water vs regular concrete, i.e., premixing the concrete with water and then pouring in hole? Since each hole is going to require about 6 bags of premix, the dry method seems much easier. Comments? thanks.