I have a 18.5"
WSM which I bought in 2005. Back then there was no such thing as a 22.5"
WSM.
If I was to buy my first one today, I would probably go with the 22.5". Yes it will use more fuel for a given smoke than a 18.5" as you are dealing with a larger volume of air to heat, but the trade off is you can get a lot more meat in there. You said you might do 3 to 5 racks of ribs. The 18.5" will need the ribs to be either cut short or bent/curved to fit. Yes you can get 5 racks in there, but not straight. The 22.5" can handle it straight across. Two 9 pound pork butts are no problem and I often do 2 per level (total of 4 9-10 pound butts) when I'm making pulled pork for the office. Can't comment on the packer in a 18.5" as I've never tried it.
You can always cook a little food in the 22.5" but you can only smoke so much at a time in the 18.5". There have been times I wished I had the 22.5" but I can't convince myself to own two (wife would make me sleep on the porch with them), or to get rid of the 18.5". Buy it once, buy it right.
There are also a few aftermarket toys for the 22.5" format that are not available for the 18.5" (remember the 22.5" Weber grill parts will also fit).
The one thing I would suggest you get no matter which
WSM you buy is a power draft. I have a BBQ Guru DigiQ 2 and would not trade it for the world, You can get different models and different brands, but they all do the same thing, regulate the airflow to mind the pit temps for you. You can also get fancy and buy a model that is wireless or network enabled. I've had mine since 2007 and again would not trade it for the world. A power draft makes an 18 to 20 hour overnight smoke easy and will let you get sleep instead of fretting over the pit all night. As long it has fuel to regulate, it should hold temps. Now don't get me wrong. A
WSM is sweet and easy to use straight out of the box. A power draft makes it as close to "set and forget" as possible with charcoal fuel though. Also once you use a power draft, you will find that the smokes are very repeatable in both time and results. Do you absolutely have to buy one if you own a WSM? No, but they are really nice to own and you will get spoiled by them real quick.
Just my take. Either way, you will be very pleased with the WSM.
Oh and one final thing. You will not have your meat taste like it was baked in a WSM. You can get absolutely beautiful smoke rings, bark and smoke flavor. And you can still us the pellet tray for cold smoking or if you want to do snack sticks (also take a look at my eWSM mod post for a removable electric element mod just for snack sticks and sausage making).