Here's a good answer -
If you want to ride trials, and are a rider who prefers riding on rocks, you might want to choose a stock.
However, because you mentioned your son now has a BMX and you want to be able to ride where he does, a 24" such as the fourplay is much better suited for BMX style riding. It's also good because if set up right, it can hold its own at trials. Dave Marshall, in fact, has done competitions on his, which goes to show how versatile it is. Plus, you can sit down!
I'd go for a seaty 24"