The main difference is in the 1st & 2nd gear ratios. The details are here:
http://www.domsat.co.uk/pages/gearboxes.htmThe 4-speed in the top picture has the standard tail case and the hybrid one in the lower picture has the GT tail case - much beefier.
Most people choose the GT box if they can find one because the standard box's 1st & 2nd are a little too low for our use. That said, we rarely use 2nd and 1st is only for starts so it's not that big deal.