Lots of issues to consider here.
First, what kind of smoker? Charcoal? Stick burner? Pellet? Electric? Each have their own advantages/disadvantages.
Lets' assume you are talking about a WSM since this is a Weber fan website. In that case, I would look at what do you want to smoke? As
@addicted-to-smoke says, if you are doing the occasional pork butt, it doesn't make much sense to get a WSM unless you want it just to have and use another smoker. I have a mini WSM, 18" WSM and 22" WSM. Each one has its advantages.
For me, I look at the length of the smoke and how much meat I am smoking. If I am doing one or two racks of ribs, I tend to use my kettles. If I am doing one pork butt, I am using my mini WSM or 18" because it is easier for me to maintain a constant temp over 8+ hours versus using a kettle (and yes, I know you can use the snake method to get long burn times, and I have done that, but I still like using my WSM for longer smokes rather than the snake on the kettle). If I am doing a brisket or more than one cut of meat, I will typically use my 22" WSM.
One downside of the WSM is it will typically use more charcoal than a kettle, especially the 22" WSM. However, I stock up when charcoal goes on sale so that doesn't matter as much to me.
Also, as a side note, taste of BBQ cooked on a WSM vs. a kettle is the same to me.
So basically, for me, the advantage of the WSM is being able to smoke for a longer period of time (8+ hours) and have more capacity. Plus, it is just fun to use.
Also, keep an eye on CL. I have found multiple 18" WSM for under $100 and two 22" WSM for around $100-$125.