OK, so I took the plunge from open-bearing bottom bracket to sealed bearing bottom bracket, and new crank.

The old spindle used nuts to attach the crank arms on the threaded end of the spindle.

The new sealed bearing bottom bracket is hollow, and I need screws. I see M5 and M8 size. What size for a generic Shimano UN55 bottom bracket? Any preference as to an actual bolt head (fig. 1) vs. one that uses a hex (fig. 2). or is it entirely personal preference?

Everything else I do needs washers so I assume these do too?