Daval: (just saw this revived topic)
The Smokin' Pro (or similar rigs) are good, and they have great capacity. However, temperature control for "low & slow BBQ" on rigs like this is much more difficult. Smoker boxes like these are designed to burn wood, and wood alone, not charcoal, which means adding fuel gives you a temperature spike. I work with a guy who has one of these and he hardly ever uses it because it is so tough to keep the temps right in the target range.
For the Weber, I have cooked as many as 8 racks at a time on it by using the rolling technique. Here is a pic of rolling ribs (not mine) with 3 on one grate.
You can roll them a bit tighter and get 4 per grate, or perhaps even 5 if the racks are particularly small. There are also various rib racks that hold them straight & on their side, but I prefer rolling. I have also cooked as many as 6 large pork butt roasts on it. Unless you are cooking for an army, the Weber has plenty of space.