Using 25% of its maximum HP, the user creates a Substitute with 1 HP more than the amount of HP lost by the user.
The substitute will break when the damage the substitute has taken is greater than the HP used to make it.
So both (Sp)Defense and HP.
HP so your Substitute has some significant amount of life & (Sp)Defense to lose less HP each turn.
BTW, an interesting point.
A one-hit KO move, if it hits, will always break a Substitute.