Well, the two numbers are so closely related it almost doesn’t matter. Rake, or caster, refers to the angle from vertical at which the steering head is positioned, generally anywhere from 20 to 45 degrees – really sporty bikes being closer to 20 and custom choppers being more like 45. But the main purpose of that rake angle is to achieve the trail that the designer is after, because it’s trail, really, that determines how quickly, and with how much effort, the motorcycle will bend into a turn. A steep rake angle will almost always be accompanied by a trail figure on the short side.
Motorcycle guru and all-around genius Tony Foale conducted some experiments back in the day, circa 1982-83, which have now found their way onto his website. Basically he modified a BMW R75/5 so he could try various rake angles, and found that changing rake from the standard 27 degrees, to 15, then to 0 degrees, really didn’t hurt the BMW’s handling at all; in fact it improved it, as long as trail remained around the standard 3.5 inches or so.
What decreasing rake did affect, and not in a good way, was braking, which suddenly became juddery – but Foale postulated that was more to do with the BMW’s flexy fork tubes than with the modified geometry. You can read the whole thing at the site linked above, under “Experiments with Steering Geometry.” Basically, his experiments prove it’s trail that really determines a bike’s steering characteristics.