Assassin is a pretty big jump into the deep end.
You really can't go wrong with either size WSM. A UDS is great, but sometimes I think it's not a great 'first' smoker only because you can learn so much from other smokers it will change how you actually end up building it! But then... you get to build another UDS so that's kind of an added bonus!
Just stay away from anything that 'feels' cheap, because it probably is. If you follow that no matter what you buy the fine folks here will be able to help you figure out whatever you get and turn out some amazing q.
Neil -- "Easy Bake Master"
NB Bandera [gone to heaven...conveniently located in Long Island, NY]
Rebel 23, WSM A, WSM B, UDS, Easy Bake Oven
Pork Pistols BBQ
MOINK...It's what's for linner.