I can't think many instances where I've used a round smoker/grill, but the new Char Broil SRG I have on order might change my mind
One reason I favor rectangular grills is for ribs - Costco sells a package of spare ribs (they call them St. Louis style, but they still have the rib tips present) that contains 3 racks per package - just enough to properly cover the cooking surface of the Traeger pellet smoker I like to use for ribs. The rectangular cookers seem to maximize the usable cooking surface area quite well.
On the flip side, my mother had a Kamado (round ceramic cooker) about 35 years ago that did spectacular BBQ chicken. I bought a Big Green Egg - Mini for the RV a couple of months ago, and I must say that it is lots of fun tending the mini while it is burning it's lump charcoal.
The bottom line for me is that I like them all, each for a particular purpose (at least that's the story I'm telling my wife, and I'm sticking to it)