Having used both over the years I'd say they're both quite adequate but a solid cradle is necessary when you need a secure way to hold a moving boat.
Aside from that, the pros of the jackstands are the same as the cons for the cradle: it's a small (big) piece of steel (steel or wood) that's easy (a pain in the neck) to move around and store for the summer.
My club happens to require cradles because we use a marine tidal railway to haul and launch boats. It's an amazingly simple and robust system but it involves supporting a moving boat on the railcar, and then schlepping the boat around the yard a few inches above the ground with a "yard car" which straddles the cradle. Can't be done with jackstands.
In severe weather, ie a hurricane, I'd greatly prefer a cradle. Jackstands, I think, could be susceptible to fail if there's an undue amount of wobbling. I've also climbed to the top of my mast with the boat on a cradle--no worries.
(I still have a nice Jowi 2.0 folding steel cradle on Long Island Sound. Without a flatbed truck or trailer wider than what U-Haul offers, it can't be moved as far as Brian K's upstate New York location at reasonable expense.)