I would like to use one of these exercises to help strengthen my back, in conjunction with stretches in my chest and shoulders to help correct my rolled forward shoulders ('computer posture'). Of these two, which is more appropriate for that purpose?
Horizontal rows are generally better for correcting posture in deskbound workers.
You should be looking to strengthen your rear-shoulder muscle-groups: Trapezius, scapular deltoid, rhomboids, scapular retractors.
Any sort of horizontal row where you keep your elbows high will do the trick. In addition, try any of the following:
- Rear-deltoid raise