I love my 18.5 inch and it is very easy to control the temps.
Let's talk mods though.
I now use the base of a clay pot in place of water as a heat sink. Wrapped in foil the clean up is very easy and I don't have nasty water to discard nor do I have to worry about refilling the water on a loonng smoke.
I also added a second charcoal grate stacked on top of the first one at a right angle. Makes much smaller holes and blocks the openings on each side of the single one that used to allow chunks to fall down. I use lump coal sometimes and it seems to keep the random pieces up longer.
I replace the door with a stainless steel door from Cajun Bandit, much nicer door and it's a heavier gage so it holds heat better.
My is the old school model so I added a therm to the top.
The ultimate add-on was a BBQ Guru CyberQ II. With this baby I can literally set it up and walk away for as many as 12 to 14 hours and never look at the unit. Great for all night cooks.
I live in Kansas to it gets cold and this winter I want to buy one of the BBQ Silicon jackets for insulation. It also extends the burn time of your fuel so that you can easily smoke for 20 hours without a refill.
I bought mine at a local Ace that has a customer appreciation day once a year. 30% off any single item in the store. There are always a bunch of people in line when they open and most are buying a Weber Gasser.