The ballasted gliders resist deceleration better than
the unballasted ones. Golf balls are another example.
If we shoot a golf ball straight up and a ping pong
ball straight up at the same velocity, the golf ball
will go much higher because it resists deceleration
better (has more momentum). Given two objects of equal
drag, the heavier one will go higher, if we ignore
the idea of the 'wing' and the need to translate horizontal
to vertical momentum. If we include the concept of
the wing, at some weight difference between the gliders
and at some selected initial airspeed, the additional
angle-of-attack required by the ballasted glider pull-up
causes enough additional drag to negate the benefit
of momentum and the unballasted glider climbs higher.
The two easy special cases to consider a 1) unballasted
glider with weight = 0 and 2) initial airspeed = ballasted
glider's stall speed. If 1 and 2 are true, both gliders
attain the same height gain of 0. If 1 only is true,
the ballasted glider climbs higher. If 2 only is true,
the unballasted glider climbs higher. For other values
of weights, initial airspeeds, and drag/lift, 'it depends...'
The easy math shown here demonstrates that all three
cases ,, and = are possible.