Interesting perspectives. Personally, when I've been in the situation I've gone for the wing just because he's usually the cleanest shot to line up.
However I noticed several of you assumed that bypassing the wingman to fire on the lead would put you in the wingman's sights. May want to review your situational awareness.
As stated above: the wingman and lead are flying rather close together (between D100-200 of each other, however there's separation both front to back AND side to side: The wingman's line of fire is clear so this is not a conga line, meaning healthy left/right separation so his guns are clear if he has a shot. The trailer may be no more than 50-100 yards back of his leader, possibly even as little as 25). Unless your convergence is set absurdly short, (100yds) even if you wait a moment for the lead to pull into convergence both contacts will still be ahead of you.