If you boost Horus' ATK by any amount, then there's not many monsters that Armed Dragon can send to the graveyard to destroy Horus (how many cards do u know that have more than 3000ATK? Not many). Horus also has higher base ATK, and Horus will be able to stop it's opponent from using equip cards to boost Armed Dragon's ATK anyway. Also, horus' effect is much more devastating, since ur opponent CAN'T use spell cards, while with armed dragon it only destroys FACE-UP monsters.
OK, I agree with u that Armed may be more of a threat to Horus without boosting anyone's ATK, but then if ur going to include other monsters (discarding cards with armed dragon's ability), then u should include ATK-boosting cards aswell. Besides, if neither cards have boosted ATK then Armed's effect would destroy Armed as well as Horus (since armed's ATK is lower).
EDIT: Oh and I can actually name a few lvl. 6+ spellcasters: Dark Magician/Dark Magician Girl, Dark Magician of Chaos, Chaos Command Magician, Chaos Sorceror. There's more but that's just a small collection.
EDIT2: And I see u've not voted for Armed anyway. ;)