I'm gonna start off quoting myself from an earlier post....
What do you currently have now? If nothing else, you might want to start with a Weber Kettle. They are so versatile. If you already have a charcoal or gas grill and you want a dedicated smoker, then a big consideration is how much food would you be preparing in a single smoking?
Will you be primarily burning charcoal and wood chips or do you have a chainsaw and log rack for feeding a smoker logs and chunks by the armload?
Willy, right now I have a horizontal smoker and would like to have an upright bullet smoker down the road too. They both have their merits. THere's also a large smoker called the Bandera that combines a large vertical smoke box with an offset firebox. The firebox also has adjustable grates for direct grilling too.
Horizontal units offer lots of room and versatility. Vertical smoke boxes like bulletsmokers let you stack meats above each other. That lets the top meat baste the lower meats with drippings.
You gotta love that.
For overnight smokings, bullet smokers also let you keep a long fire going with a technique called the Minion method. Its a lot less maintenance. Not that you should ever leave your smoker unatended, but it could mean the difference between tending a fire all night and snoozing in a hammock while it burns in the background.
Another consideration is how often you want to replace it. If you know what your doing, food cooked in a $30 charbroil bullet smoker should taste the same as food cooked the same way in a Weber bullet but I wouldn't expect the charbroil to last nearly as long.
Hope that helps.