.058" will carry more oil down the bar. There's a little more meat on the chain depending on brand also. I run a lot of .063" but no .058" because of the lack of chain options in it. The .050" is standard in the southeast and .058" is mostly northeast/midwest/canada. The rest of the world uses mostly .058" husky stuff and .063" stihl.
I don't bother with fancy bars for normal use as I get maybe a season out of a plain Jane Stihl bar before its beat. At around $40 a pop it doesn't break me. No reason to have longer than generally needed. No reason to run a big bar "just in case" if 99% of the cuts don't need it. Keep the big bar handy, run a small bar! I run an 18" which I find is a good balance of cost, weight and length for my uses. I find while a longer bar might mean less bending it also means many cuts without being about to use the dawgs. I'd say for most folks a 16, 18 or 20" bar will take care of most firewood.
