The combi has a bigger footprint and two, side-by-side compartments under one hatch. You can use one as a refrigerator and the other as a freezer OR, if you like, you can set the freezer one to 40 and effectively get extra refrigeration space.
I am reading tons of reviews and the 45 with the top - I think it's the one you are thinking of - seems like a better use of space for me.
Here are some links:
The combi, big and a lot more money: http://www.engel-usa.com/mt60combi.htm
The 45, along with the two zone top: http://www.engel-usa.com/twozone.htm
On all the boards, the Aussies rave about Waeco, and you can get Waeco/Dometic portable units in the US from marine dealers. Pretty reasonable, too, but they don't seem as overbuilt as the Engels.
I was looking at the ARB unit, but am not sold on them.