In many people's (mine included) opinion, Benelli is the best shotgun made for competition, combat and defensive purposes. It even excels at trap (or was it skeet Rob?). You should be aware that the shotgun will have much more recoil than either of the weapons you've mentioned above.
Shot is basically small pellets that are carefully packed into a single shotshell. Sizing ranges from extremely small (9 - dia.=0.08 with 409pellets/ounce) up to various sizes of buckshot (0, 00, 000 dia.=0.32,0.33,0.36 with 140,0115, 98 pellets/pound respectfully). The quarry usually is the deciding factor in determining what loading the shooter will be using.
Slugs are a single large projectile that is loaded into the shotshell as opposed to shot (the pellets). This makes the shotgun more characteristic of a rifle in that a single projectile leaves the barrel after each pull of the trigger.
The reasoning of shooting slugs with a special barrel had to do quite a bit with chokes. Chokes are constrictions built into the barrel near the muzzle that would help direct the shot pattern into the desired concentric sizing. A tighter choke (full, etc) would result in a smaller pattern, i.e. the shot would stay closer together after leaving the barrel. Conversely, a looser or more open, choke (improved cylinder, etc) would allow the pattern to more fully open up . This would allow the shooter more lattitude in pointing (sort of like aiming) thus directing where they wanted the shot to go. Slugs could not get by some of these constrictions and would sometimes destroy the barrel, etc. With most shotgun barrels coming out nowadays there are screw-in chokes fitted to the muzzle ends of barrels instead of being permanently formed within the barrel itself. Thus the shooter need simply only to change the choke insert to be able to shoot slugs instead of buying another barrel.
Can't think of any shotgun web sites offhand that would have the info you're looking for but several books have info on the sizing of shot and it's intended uses. Cartridges of the World has shot size info. Will look and see if I can find a website for you.
Hopefully someone else will add to the many things I've skipped over.