It is true that on a vertical door you lose some of the cold air when you open it. But air has very little heat content, so the energy lost is quite small. The biggest contributors to energy loss in a fridge (more properly heat gain) is through the walls and weaknesses in the walls, and through adding warm drinks and food periodically. In typical use these are about equal. So said the Glacier Bay technical folks, and my experience has proved them right.

Quick envelope calculation: 6 cu ft box x specific heat of air 0.240 x density of air 0.076 x 40 deg (room temp - refer temp) = 4 BTU to cool the air if all of it is exchanged during an opening (worst case). 12 oz drink / 16 oz/lbs x specify heat of water 1 x 40 deg = 30 BTU. So the box can be opened and the air completely exchanged at least 7 times to the same effect as cooling one beer (or 45 times for a 6 pack).

If you have a front opening door with bad seals, it will continuously lose cold air in a thermal siphon. This won't happen as readily with bad seals in a top door box, so it's more important to have a good seal and latch on a front door.

If you are going for every last bit of efficiency, then top opening will help a very small amount. But I think the main advantage of the Engel is better insulation, tighter seals, etc. Engel makes front door models as well, if that fits the space you have.