I just got the 22.5 about 3-4 months ago. I do not have the 18.5 so hard to compare the 2. I would say I think if I had the 18.5 it would more than meet my needs 95% of the time. Only reason I got the 22.5 is at the time Amazon had a $50.00 coupon for all grills over $300 and this was included, so it was just a $50.00 difference between the two.
Now I will tell you quite honestly when I got mine from Amazon and went to open the box I think the next day the cooking grates that lay across the top were pushed down into the center section at an angle. After watching the unboxing video on this site I thought that was not quite right.
Sure enough, when I went to assemble, the lid (which was in the bottom) would not fit on the center section. Not even close. was not a bear hug adjustment issue, but was pretty far off.
So I have used Amazon for years and never had an issue, but in this case YES I actually did. Was it Amazon? Weber? UPS? who knows but somewhere along the line something happened to the box or one was placed on top with enough force to bring the center way out of line from the grates.
i DID call Weber and explain and to their credit Weber immediately sent me out a NEW center section as well as grates. I know it might of been amazon's fault, but to return to Amazon I would of had to mail the entire unit back when I thought only the center section needed replaced, and they would of just sent back to Weber (I think) so I thought this was the best solution anyway.
NEW center section works great.
As far as the temp probe thing - it is great. I have found the probes go through much better through the longer slot then the smaller holes. there is no leaking and issues
finally maybe I got lucky with the new center section, but the door and center section aligned very well and I get very little smoke. I did not even adjust the door.
hope this helped