also, don't underestimate the subtle, but noticable (and in my opinion critical) flavor imparted by using charcoal, let alone wood smoke.
Another thing about WSM is that it is very efficient in its smoking ability, in that the smoke is carried directly over, around, etc. the meat as it passes up. In a gasser, if you use wood chips, it takes quite a lot to get the right kind of smoke to really flavor the meat. It usually mostly escapes before it goes anywhere near the meat. While offset smokers have the same horizontal set up, offsets are designed for proper smoke/air flow to maximize that smoke passing the meat, etc. (intakes are below the grate, single our double outtakes are above, with vents, flanges, etc., to completely control the movement of smoke. In a gasser, they just aren't designed to do that.
Last thing I'll say is that unless you have a big gasser, I have a hard time getting even heat in it. The part closer to the heat source is 50 degrees hotter than the part on the other side. So, for even cooking, I've got to flip the sucker every half hour. Considering I don't have to fiddle at all with the WSM once it is started (it maintains it's heat so well I don't touch the vents even) having to flip, add wood chips, etc. negates any convenience benefit from doing it on a gasser.
All that said, it doesn't mean you can't produce good slow cooked Q in a gasser. I just find it easier on a WSM and the end result is consistently far better.
