Help with corner to corner join

If you wanted to weld two plates of 3mm steel with an arc welder in a corner to corner weld. After you do the tac welds.

Which size electrode? 2 or 3mm? Would the 3mm be too thick & hot?

And what gives better penetration, a straight weld down parallel to the sides & maybe a few more runs? Or across in zig zag? Would this 2nd way really distort the straightness of the seel compared to the straight down runs?

thanks