I'm not sure if defense v. offense would be better, but your idea is certainly better for two reasons:
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8% AD/8% AP is the equivalent of 5 tier 4 mastery points (Warlord/Archmage) - giving your opponents' team 20 mastery points. Your idea would be about 4 tier 3 mastery points (Juggernaut)
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A select group of champions (hybrids or champs like Teemo/Kog that use AAs+ spell damage) benefit from both sides of the buff, giving them 10 extra mastery points instead of 5. Your idea would make the bonus more equal among all champions (although it would help health-building champs more).
Oh, the above understates the power of the first buff. Warlord only boosts bonus AD, dragon boosts total AD, so it is still useful for mage's that don't get any AD. Honestly, I think I'd like to see a flat AD/AP bonus that scales with champion level.