I have a big reverse flow and a small traditional offset. I put a plate in my small offset to even it out a little, but I like the 10-15 degree variance in the little cooker. I use that as I point chicken legs at the hotter end or the brisket point at the hotter end, etc. As for chimney placement I think chimney diameter is pretty important also. In my big reverse flow offset the chimney diameter is huge so it moves air quickly. In my little traditional the diameter is not as big (comparatively). But I think this is OK because I use lump charcoal often as the "heat" in the little guy whereas the big one is always a roaring fire.
I like the theory (so does Aaron Franklin) of the chimney opening being at the grate level as opposed to the top corner of the cooker. I do not think this is a factor in my small offset because the cook chamber is small enough that everything is getting hit with smoke. If you're only getting one I would get at least a 20" diameter cook chamber. My 16" is great for me only because I have big ones for when I want to cook for a big group. I would also consider what is made around you. Shipping can be 1/2 the price of a cooker. (that's how I ended up with a Bubba over a Lang)
Finally, back to your original question, if you are getting a big cooker I would recommend a reverse flow with a big chimney. If you are getting a smaller one I would not go below 20" diameter and decide if you want a really even cook chamber (reverse flow) or if you want a little variance you could go traditional. (I'm sure someone is about to say they don't have any temperature variance in their traditional offset, this is just my 2 cents)
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