Gust is an impulse load on an aircraft: a sudden change in force, which distorts the previous equilibrium. Upon a vertical gust, the aircraft will respond with a second order response: vertical acceleration, damped by the air resistance.
The factors for accelerations due to gusts of a certain intensity and duration, as per this answer:
- Elasticity of the wing construction material acts like a spring. Load it with a vertical gust, and it will bend upwards, then spring back.
- The wing bending experiences damping forces from the surrounding air, proportional to bending velocity.
- The wing sweep angle spreads out the gust: not all of the wing is accelerated upwards immediately.
When comparing the two planes (from the wiki pages):
- Faster planes are less impacted by a gust load - the B707 cruised at 1,000 km/h, the 787 at 900 km/h, so this is in favour of the 707.
- Wing sweep is comparable, 35° for the 707 and 32.2° for the 787
- Higher wing loading is favourable: 151,300/283 = 535kg/m$^2$ for the 707-320; 254,011/377 = 674 kg/m$^2$ for the 787-10, the 787 wins
- Higher aspect ratio is favourable: A = 7 for the 707-320, 9.59 for the 787: it wins
Simple observation as a passenger reveals that the carbon-fibre composite wing of the B787 flexes up-and-down a lot, so the spring-mass-damper response of the B787 would be slower, despite its 10% lower cruise speed.
Screenshot from this video
The active gust alleviation system of the B787 deflects the inboard flaperons as a function of detected vertical acceleration, which indeed helps in comforting out the ride.