5% spell speed would increase your spell speed stat by 5% which would increase your HoT potency a little and make your spell speed on the stat sheet a little better looking
but 5% attack speed reduces all your cast times/gcd by 5% which is simply a far stronger effect

using the new 3% attack speed fey wind is a little stronger than the old 30% versions as a reference
should mean 5% attack speed is 10+ times better than 5% spell speed would be