If the line is cut to the right length and using the right technic, the righting moment is the same.
However the forces to hold on the line are higher if attached under the tramp, maybe so high that is hard/impossible to hang low enough to get the boat up. Also your fore aft movement is limited and makes it hence more difficult to keep the bows windward. Making some loops in the line, where you want to hook to the trapez helps a lot.
In conclusion if you are not so heavy, the over the hull method is easier to use.
It is also not stupid to have a long line attached to the mast and easy to grab instead a short one under the tramp where you don't get it if you or somebody else need it most.

Cheers,

Klaus