Are you sold on gas or is electric an option? From the questions you asked it seems your biggest concern is either horizontal or verticle gas.
Most smokers have some means of putting a water pan between the heat source and the food. Water pans add moisture and stabilize the cook chamber temperature by acting as a heat sink. Burning gas generates a lot of moisture so some of the gas burners may not have water pans. Many members will also add flavorings such as beer, spices to the water pan. Some members will go a step further to stabilize cooking temps by filling the water pan with sand.
With either gas or electric you need a way to generate smoke. Either wood chips, dust or chunks. Whatever smoker you decide on make sure you are happy with the smoke pan or tray. That seems to be the most difficult part of cooking with watt or dino fuel burners, getting the wood chips to burn properly.
You want to ask about recovery time, how long it takes to get back to temperature once you fill the smoker with meat. Some of the less expensive smokers have problems generating enough heat to power through a full load of butts to recover quickly.
The difference between verticle and horizontal is less clear to me. Sometimes it comes down to how much space you have, how big a smoker you need and how much money you want to spend.
I think you need to remember one thing about this hobby. Once you get into it the smoker you have will become to small, to slow to something. It will not be long until you start thinking " I wish I had bought the ------". It is a perfectly good option to start small, see if you like the hobby, find out what you want in a smoker and then make your major purchase down the road.
