If you are going to build a cart I would use the 22" because it has the most widely used accessories and will be your daily work horse. The SJ's can always be used on the table top if you need extra capacity or just need to cook a small order of food.
I am actually not a big fan of wood carts myself. A lot of work to build, usually aren't waterproofed as well, and you can usually find a nice Performer with gas assist in the $100 to $150 range. I do think having additional tables/cabinets are nice if you have a nice dedicated work space though. I am actually on the look out for a cheap local Genesis frame to turn into a table/cabinet.