Before this I had a large BGE for about 10 years. That has an 18" grate and I found out that in 10 years I could only recall twice needing more space than what I had. If you look at the comparison between 21" vs 23" KKs the difference on main grate size is so small that once in a blue moon that I might need more than 21", the 23" will not be the answer (the 32" would, but that is a completely different ball game).
With this information at the back of my head, I called Dennis and had a conversation with him, which reinforced my initial thoughts. Therefore I used the money difference between the 21" and 23" to get a bunch of accessories for my 21".