Just some additional considerations for prospective buyers...
The MT45 has the top hinged along the short side, so it's end opening.
The MR40 has the top hinged on the long side, so it's side-opening. I ended up reversing the lid opening on mine so that I could keep the cooling holes and power wires open at the correct end of my install. Side opening has worked out better for my install.
It's easy to reverse the top, but you lose the "Engel" label on the side.
Just for the record, MSNomer is the "cool" one who started it all. I bought mine based on her glowing recommendations! I continue to be impressed at how long it will run on my batteries. At the prices offered, it's a no-brainer!
I ended up getting the 45 -- a little more room inside and as you mention, hinged along the short side. I think this will work out better for my layout. I won't have to pull it out all the way to open the lid.
Can you box these in pretty good inside of a closed cabinet or is airflow/cooling a big issue?