[dar512]

If you only wanted a smoker, then I'd go vertical. The temperature will be more consistent over the whole smoker. However, since you want to grill on it too, I think you need to go horizontal.

If you go vertical you get Pi*24^2 or ~452 square inches of grilling space. If you go horizontal you get 24 * 48 or 1152 square inches of grilling space.

You can compensate for the drawbacks of the horizontal design through certain tweaks. Do a net search on "silver smoker mods". That'll give you some ideas.

BTW - Smoking and bbqing require pretty much the same design. The only difference is the temperature.