Well, since I have 2 of the 3 units you're looking at, perhaps I can address some of your questions. Leakage? Yes, some but not detrimental to the mission. If it's smoke leakage, you want a thin blue trace gracing the meat anyway. If you've got lots of smoke escaping, you're generating too much smoke anyway. Heat leakage, they're propane, turn it up a tad and you'll be ok. You don't say where you're from so I have no way of knowing if you'll be trying to smoke in zero temps during the winter or sweltering in the deep south heat.
Yes, the Master Forge is half the size, in width of the GOSM Big Block (BB). You want to smoke big things like briskets and the like, you got no choice but to go BB. But, then it may not be big enough either. I've had to cut full rib racks in half to fit properly in the BB. The Master Forge is my traveling rig. It cranks out the food very nicely. Has the advantage of 2 doors, so when you're adding chips in the lower section, you're not dumping heat out the top which is a problem with the single door GOSM.
Second: The Master Forge is a great starter smoker if you're new to this game. You may get down the road and discover that you'd rather go down the WSM trail (don't ask, I'm on that trail right now, kinda sorta and don't ask about that either, it's too long a story for your needs right now), you might just want to get the toes wet before jumping in with both feet.
I'm kinda the type of guy that if I can't slam the doors, kick the tires and look underneath the car, I'm not buying it. Master Forges are readily available, the last time I looked, through Lowes. You can get a good look at one and see if it's going to fit your needs. Sometimes when you order via the internet, it may not be what you invisioned it to be.
Don't get me wrong, I like the BB GOSM as well, but it's too big for my summer vacation type trips.
Now if you got the bucks for a BB GOSM, you've got enough to look at a WSM 18 incher. Now there's a smoking machine, not propane. Lots of good deals out there for the 18 incher and we've got a lot of "experts" (don't ask how I know this) on this site more than willing to discuss the pros and cons of that system.
Good Luck in your searchs.