Which would cause more damage - a bus moving at 25 mph or a small car moving at 25 mph?

It would be the bus moving at 25mph

Talking in terms of momentum, we know that momentum is conserved in a collision. Something that will cause the most 'damage' is something that posses more energy or momentum

p=mv

with velocity being the same, the object with the greater mass (the bus) will have a larger momentum and will thus cause more damage.

A bus because the wheight of the bus is a lot and the length can hit more care's or even kill people