I have the KK 23â€. The KK 21†wasn’t available when I bought mine last year. But I think I would still pick the KK 23†if I had to do it all over again. Cooking for just my family of four can be done with either grill fairly easily. Cooking small amounts of food looks silly, but the KK is so efficient that I don't mind lighting a full basket of charcoal for a quick cook. If I was super anal about that, I would use my basket splitter.
We will cook for friends several times when the weather is good. But the times we’ve had parties, it was a lot of cooking over direct heat (burgers, hot dogs, etc.), and I used up all of the area of my main grill grate pretty easily. If our parties were more along the lines of “make 6 butts for pulled pork all at once for a crowdâ€, I might consider the KK 21â€. Of course, you know your cooking and entertaining habits better than anyone.
Even with the high-cap lid, the KK 23†gives you more cooking room, and there have been enough times that I was really happy that Smaug was as big as he is that I can’t imagine wanting to go smaller.
You could make the argument that the KK 21†gives you a better price-per-square-inch value, but the KK 21†is still going to be smaller. The price difference between the two is $560-580, depending on the type of tile, and for a lifetime purchase, that price difference is inconsequential. As I like to sometimes point out, if $580 is going to make or break a grill buying decision, then you can’t afford the meat that’s going to be cooked on your grill.