I'll take a swing at this one...I purchased a 18.5 OTP about a month ago. Grill works great, but it is tight getting 4 split Chix breasts on there cooking indirect. I'm actually upgrading to a Performer for the additional cooking space. I have an 18.5 WSM that I use with no space issues since I can utilize the entire racks due to the design.
If you are planning on just using HH for things like searing steaks, then you probably will be fine.