I can't help you on the mechanical differences, I've only been introduced with this version.
But the bestiary in the core book is pretty short. You'll really need the Old World Bestiary to round out a much fuller selection. The core book gives you the Brute, Sneak and Chief careers. The creatures it gives are Beastmen, Daemon Imps, Goblins, Lesser Daemon, Mutants, Orcs, Skaven, Skeleton, Wight, and Zombie. Common animals include Bears, Destriers, Dogs, Light Warhorses, Ponies, Ravens, Riding Horses, Wardogs, and Wolves. Common NPCs include Bandits, Beggars, Blacksmiths, Footpads, Gamblers, Pickpockets, Proprietors, Rakes, Sell-swords, Town Guards, and Wreckers.
That's all.
The Old World Bestiary covers the new careers of Apprentice Shaman, Shaman, and Shaman Lord. We get the creatures of Banshees, Beastmen (bestigors, centigors, gors, and ungors and brays), Chaos Dwarfs (and bull centaurs), Chaos Marauders, Chaos Warrios, Daemons (bloodletters of Khorne, chaos furies, daemonettes of Slaanesh, horrors of Tzeentch, plaguebearers of Nurgle), Dire Wolves, Dragons, Dragon Ogres, Dryads, Elven Corsairs, Fenbeasts, Ghouls, Giants, Giant Rats, Giant Spiders, Giant Wolves, Goblins and Night Goblins, Great Eagles, Griffons, Harpies, Hippogriffs, Hobgoblins, Hydras, Manticores, Minotaurs, Mummies, Mutants, Ogres, Orcs (black orcs, common orcs, savage orcs), Pegasi, Rat Ogres, Skaven (clanrats, clan Eshin Night Runners, clan Moulder Packmasters, clan Pestilens Plague Monks, clan Skryre Skirmishers), Skeletons, Squigs, Snotlings, Spirits (ghosts, poltergeists, spectres), Treemen, Trolls (common trolls, chaos trolls, river trolls, stone trolls), Unicorns, Vampire Bats, Vampires (blood dragon vampires, carstein vampires, lahmian vampires, necrarch vampires, strigoi vampires) Warhawks, Werecreatures (were, child of Ulric), Wights, Wild Boars, Wraiths, Wyverns, and Zombies.
Plus these mounts, Chaos Steeds, Daemonic Mounts, Destriers, Elven Steeds, Light Warhorses, Nightmares, Ponies, Riding Horses.
Plus these additional animals not covered in the core rulebook, Prey Animals, Small Raptors, and Snakes.