Small,
I too graduated from a BKK to a KK. You can see the BKK lurking in the background. The 21" wasn't available at that time, but I think I would still go with the 23". I love the extra real estate. I'm only cooking for three, but having a good sized footprint makes reverse searing using the basket splitter a breeze. It is also great when you are cooking meats and a bunch of veggies. I use the basket splitter for most of my cooks and the 23" gives two large zones. One for direct and one indirect.
I'm sure you will be happy either way. The real question is what are you waiting for? My only disappointment upon receiving mine, was not buying it sooner.