For 100yd and the occasional stretch out to 200, I'd go with the 18.5" hands down.
The biggest advantage of the 24" barrels is a longer powder burn and increased gas pressure to propel lighter 45-50gr varmint projos out to longer distances. We're talking 400-500yd before you begin to realize any real benefit. One disadvantage of the longer barrels is that the increase in length and overall surface area, as Water Monkey pointed out, makes the barrel more susceptible to changes in thermodynamics and resonance. These factors can have some dramatic effects on shot placement.
For short yardage, out to 300yd, the 18.5" barrel will be plenty effective. The shorter barrel will be more rigid and less affected by changes in heat and resonance. It's also lighter weight and easier to maneuver overall. The 20" barrel is a good length as well, but the 1.5" of added barrel won't make much difference at all on target...