We have had both and they both work very well. Doyle makes two versions: the original StakPac and the Cradle
Cover. We have the Cradle
Cover on this boat and the Mack on our previous boat.
Advantage of the Mack is price
and easier to stow against the boom when the sail is raised (if that is important to you) and it is not permanently connected to your sail.
Supposed advantage of the StakPac is that the membrane sewed to the sail pulls the canvas
against the sail when raised and filled. This seems like a specious advantage to me, with several disadvantages of having the StakPac permanently connected to your sail outweighing it.
The advantages of the Cradle Cover is that it is not permanently connected to your sail and the lazy jacks are connected to full length rigid battens running fore and aft on each side of the opening. This makes for much neater and easier lowering of the sail into the pack vs. the Mack, and it contains the sail better. However, it is not as easily stowed against the boom when the sail is raised as the Mack and costs about twice as much.