Really depends on where you are ability wise in skiing. If you are only somewhat comfortable with 7's, cork 3's are probably easier to learn. If you're really comfortable with regular spins up to say 9, a cork 7 is much easier. For a lot of people that can cork 3 and 7, its generally easier to cork 7 because you set the spin more off the lip with first your pop, then your upper body/arms and it comes around perfectly. A cork three on the other hand requires a lot of hip thrust (yep, I said it), which is a lot different than other tricks.