The WSM is great cooker right out of the box. The lesser expensive offsets will need some modifications to get them to run more efficiently although they will turn out fine product. Heat retention/insulation will affect how fussy/how much attention they need. The thinner guage offset cookers are more sensitive to wind and low air temps. The better quality offsets are made with heavier steel, 1/4" thick. The thicker steel helps hold heat and maintain steady temps during the cook and so the temp in the cooker does not drop dramatically everytime you open the door to mop of if your fire/charcoal cools.
The typical plusses for the offsets are that you have a larger cooking area, easier access for mopping and removing/adding meat during the cook, and (depending on the model of upright) easier access to the firebox. Cabinet style smokers offer the same access and are usually insulated.
In any case, make sure you have easy access to add fuel (hot charcoal/wood) during the cook. having another BBQ grill running for a source of hot coals helps keep steady temps during longer cooks.
You'll find that most of the higher priced/better designed cookers are designed to keep temps steady longer and require less attention in the firebox.
Barbecue is like a coquettish young woman, favoring only those suitors who ply her with considerable time and attention.
- Bob Garner