What is the application? what is cold weather temp?
10.9 is more stronger
Cold weather will shrink both the bolt&nuts and the components attaching, if you ONLY worry about loosening consider using spring/lock washers.
The first digit is the ultimate strength of the material and the second is the ratio of yield stress to ultimate strength. Grade 8.8 bolts have an ultimate strength of around 800 N/mm2 (MPa) and a ratio of yield/proof stress to a yield strength of 80% i.e, 640 N/mm2 (MPa).
Extreme cold weather would require special bolts, I guess.