An excellent comparison between the Rolex Exporer and the GMT. Kind of like watching two terriers fight to the death.
The Explorer II is created for cave-dwellers, when working under the surface over longer periods of time they can loose the sense for day-and-night orientation, and therefore Rolex created a watch which always displays the 24-hour notation of time as an extra feature. They introduced the white dial lateron on the ref.16550 so they can see the watch better in difficult and dim light-conditions. The Explorer II used the inhouse 1575 caliber when the watch initially was introduced. The Explorer II also came with the cal.3075 and the cal.3175. The bezel is fixed and it is engraved with the 24-hour track and made from solid stainless steel.