I have a 30 and a 40. 30 was the first purchase and as said above I hated cutting up meats to use it. It is perfect for fowl though which is my favorite smoke. It uses less electricity and less smoke required to make an impact upon the meat.
After starting to play with bacon and sausage and learning the best place to buy a MES40 (oddly enough it was from Masterbuilt direct). I bought one as well as a Masterbuilt Electric Cold Smoke attachment. At a HUGE discount over anyother offered price pplus I could get exactly the model and generation build I wanted shipped directly to my door. I also learned how much easier any warranty work would be when you bought directly from them also.
I love my MES30, matter a fact I have a butt smoking in it right now. For everyday smokes its hard to beat. BUT when you get to learning bacons and hams and sausages, etc that MES40 is sweet too.
I have a AMPs and a MECS (Cold Smoker), both have there purposes, both work as specified, both can be finicky and both cost about the same.
I have been smoking awhile and for the life of me I don't understand a window, its like a screen door on a submarine. But it is fun to clean it, I would guess it losses heat since I can't lay my hand on it, but some folks do like looking in.
If only one, I would go with the 40 and I would spring for the cold smoker attachment for it strickly for the quantity of smoke delivered. Besides the free frt when purchaced with the MES40. Then soon afterwards a AMPs would be at the top of my list, (its cheaper than a tank of gas). I still sometimes have AMPs problems, it normally works, but when it starts out hard to light, its a pain and doesn't get better. Course the MECS the chips can and do stick up in the chimmeny.
Its all about understanding and some day hoping to master your equipment before you get mad enough to throw it out! LOL
OH I highly recommend you call the MES 800 number and check price and availability before you purchase. I got my MES40 w/ MECS sizably cheaper than I paid for my MES30.