Hey Kevin. I am just south of you in Huntsville AL.
I recommend you ck out The Virtual Weber Bullet for the "how big will fit" answer as well as other questions about cooking a turkey on the WSM.
As far as my preference - I like one big T-bird for no other reason than I just have to be concerned with one prep, cook, carv, etc vs two. I'd cook that 20# bird with the quickness. I have done two before and it starts turning into work pretty quick ...but of course with two birds you can try two recipes: brine differently, fill the cavity with different herbs, spices, and fruit/citrus/veggies, rub differently, smoke with different woods, etc.
However, Cook's Illustrated magazine says, "We find that the larger the bird, the higher the likelihood that it will be overcooked. With its delicate breast meat and tougher legs, a turkey is already hard to cook. But if the bird weighs 22 pounds, it's very difficult to get the meat close to the bone cooked through without causing the outer layers to dry out." Cook's recommendation—instead of cooking one large turkey, cook two turkeys weighing 12-15 pounds each.
I hope this helps. Cheers!
Dwain,
That Cook's Illustrated quote was what made me question my original thought to do one bigger bird. We ended up deciding on a 19.5# bird that we bought tonight. My girlfriend was worried that there wouldn't be enough leftovers for sandwiches the next day, so I ended up buying a 5# turkey breast as well, to cook just for leftovers. This is my first thanksgiving with the WSM. I've smoked turkey's the last 3 years, but am excited to try one on the WSM.