I suggest you EMail Todd with A-MAZ-IN. BUt... I will probably get it all wrong by attempting this but here goes.......
To me the difference between the pellets and dust is the temp. Dust is better for cold smoking because it does bot require as much air, so you don't need a "draft" thru the MES caused by the heat escaping. Pellets give off more heat or so I have been told. I know when I was trying to cold smoke with pellets I did have some problems keeping it lit. When I set the unit to 100 degrees and developed a draft thru it worked fine. Also when I was trying to hold the 100 degrees on my bacon, at night, my temp due to the help of the additional heat from the Pellets got up to 137 degrees when I caught it.
If you are going to cold smoke, that is less than 100 degrees the dust is preferred, 100+ degrees use the pellets. I really wish I had a smoke tube, I think its probably my next investment.
Yes some folks use both together or two tubes especially with bacons to get a heavy dose of smoke. I tried lighting both ends of the pellet tray and that's when my temps took off. I was trying to beat how it was designed to work.
Hope that helps, hope I didn't steer you wrong. But if you are in fact going to truly do cold smoking the tube w/ dust is the way to go. I got the tray and pellets, figured when I got into cold smoking I could afford the tube w/ dust. I Have not really done anything that either the tray or the chips couldn't handle so far but I have done little or no true cold smokes.
If you are going to use it more as a aux smoke generator when smoking with your new MES40 go with the pellets and the tray.
Good luck and hope I didn't just confuse you.