I'm with Craig on the
WSM. If you could only have one smoker it would have to be a
WSM in my opinion. Mine is a 18.5" old style (smaller water pan, no factory thermometer, etc... bought in 2005). Out of the box, the
WSM is a excellent, durable, and easy to use smoker. Add a BBQ Guru or other "power draft" (a computer controlled air flow device) and you have as close to set and forget as it can be with a fire pit smoker.
On the other hand, like others have said, the MES and other similar electric smokers excel in other areas such as the smoking of sausage. You can hang sausage/snack stick/etc... and get much lower temps than most people can achieve in a
WSM with the easy of digital settings in an electric element smoker. There are a lot of MES smokers that the display temp is off from the actual temp, but once the off set is known it's pretty easy to allow for it (example, display shows 255* but actual pit temp is 225*, so when you want to smoke at 225* set the MES for 255*). Also depending on where you live (apartment, condo), you might be able to use an electric smoker with a pellet/sawdust tray but not be able to use a charcoal smoker due to the fire pit.
For the reasons above, I want to convert a food warmer to a large electric insulated smoker cabinet. I have a donor cabinet, just need to get off the fence on if I'm going to leave the insulation "as is" or go down the nightmare road of pulling the cabinet apart and hacking out all the foam (remember my cabinet was made as a warmer and not a fridge so the temps of 225* are not outside the realm of normal for this cabinet - settings go to 200* on the unit). Sort of like a oversized MES with a PID controller.
As to taste difference in the meat from either. I'm not sure as if you use one of Todd's pellet trays or tubes in the MES you can produce some mighty fine meat. WSM works better with fist sized chunks of smoke wood as opposed to chips or pellets and also produces mighty fine meat with a great smoke ring. The pellet trays and tubes do not work with the WSM (trust me, I and others have tried. It just does not work unless you are cold smoking with no fire in the fire ring).
In summary the WSM is pretty foolproof for a charcoal smoker, and the MES is pretty foolproof for an electric.