There are a couple of threads on BGG thrashing this out...
Nathan on this thread
http://boardgamegeek.com/thread/912427/two-core-copies-vs-to...
Posted this (probably ones of the clearest assessments of the difference I've seen):
did a breakdown of what you'd be have with 2xBase vs Base+Cores (based on card lists taken from the file section). I should probably upload my spreadsheet at some point. But for now, here is a list of the extras you'll get each way:
With Base game + Core Tomes 1 & 2 you'd get the following over someone that went with 2 base games:
1 Hand of Bim-Shalla
1 Jinx
2 Purify
1 Regrowth Belt
1 Shift Enchantment
With two Base games you get the following over someone that went with base + cores:
2 Agony, 3 Asyran Cleric, 1 Banish, 1 Battle Forge, 1 Bear Strength, 3 Bitterwood Fox, 1 Blinding Flash, 3 Block, 1 Blue Gremlin, 2 Call of the Wild, 1 Chain Lightning, 1 Chains of Agony, 1 Cheetah Speed, 1 Cobra Reflexes, 1 Dark Pact Slayer, 2 Darkfenne Bat, 1 Darkfenne Hydra, 3 Decoy, 3 Dispel, 4 Dissolve, 1 Divine Protection, 1 Drain Life, 1 Eagle Wings, 1 Electrify, 1 Elemental Cloak, 1 Emerald Tegu, 1 Enfeeble, 2 Essence Drain, 1 Evade, 1 Explode, 1 Feral Bobcat, 2 Fireball, 2 Firebrand Imp, 1 Firestorm, 1 Flaming Hellion, 1 Force Push, 1 Gray Angel, 2 Harmonize, 1 Hawkeye, 1 Heal, 1 Hellfire Trap, 1 Highland Unicorn, 1 Jet Stream, 1 Knight of Westlock, 1 Knockdown, 2 Lightning Bolt, 1 Mage Wand, 3 Mana Crystal, 3 Mana Flower, 2 Mana Leech, 1 Marked for Death, 3 Minor Heal, 1 Mongoose Agility, 1 Moonglow Amulet, 1 Moonglow Faerie, 3 Nullify, 1 Pacify, 2 Pillar of Light, 1 Poisoned Blood, 1 Purge Magic, 1 Retaliate, 1 Rhino Hide, 1 Ring of Fire, 1 Royal Archer, 1 Sacred Ground, 1 Seeking Dispel, 2 Tanglevine, 1 Teleport, 1 Teleport Trap, 1 Thunderift Falcon, 2 Timber Wolf, 1 Turn to Stone, 1 Vampirism, 1 Wall of Fire, 1 Wall of Stone
You'll also get additional spellbooks, bits, markers, an extra board, ect. So if you paint the extra markers you'll have all you need for 4 players with a bigger arena. And you'll also have two of each Mage so you could do mirror match-ups if you wanted.
[Edited to correct errors from the number counts I was using.]