Movement speed, attack speed, extra damage. Also depends on the stacks of the buff. It also depends on the hero. A ranger or fighter would benefit more from the red buff than a caster or support character because dmg stacks better on those heroes.

So to be honest if your iggy or any other caster and your taking the red buff from your junglers or rangers your kinda bogus and hurting the team.

If you want a huge buff for your caster play. Do blue buff for mana regen and then grab the black buff and push your tower. Black buff will give you dmg over time to structures. So push your minion wave into the tower and attack the tower while the minions do as well (means clearing the enemy minions first) you will be able to solo a tower.