If you're in the market for a rig that can handle both grilling and smoking, here are things to keep in mind:
Design: The firebox should be large enough and positioned properly to distribute the smoke and heat evenly throughout the cooking chamber.
Construction: The better smokers have metal fireboxes and heavy metal cooking chambers.
Ease of cleanup: Look for an ample ashpan or other mechanism.
Cooking space: I don't care about the secondary shelf, the one that goes atop the basic grate, because I don't like meats dripping down on the meats below them, even if they are the same (brisket above brisket, say); it makes the meats below greasy and softens the crispy exterior that, in barbecue circles, is known as bark.
Size: There needs to be enough space to fully open the cooking chamber's lid and its firebox.
Cost: You can spend anywhere from a few hundred dollars to more than $10,000.
Adjustable grate: It brings the fire closer to a steak for a good char and can be used to position foods farther away, such as a gentler treatment of zucchini.
Cooking surface: Do you need enough space to feed a party of 20 or more? Or will you be cooking mostly for the family and small dinner parties?
Style: I grew up using an offset smoker, and I prefer it to vertical boxes. But box- and bullet-shaped smokers take up less room. Lots of people swear by them.