what is the advantage of aluminum heads over cast iron heads?

Weight, repairability ease, and a bit more selection of varieties than cast iron heads, especially in more race-oriented applications. It is a lot cheaper to weld up a hurt aluminum head that to try to get an iron one fixed, if it even can be repaired. You can easily replace guides and seats too. If you have a lot of money into them, they will last you a long time with their plusses over iron heads. The only downside may be initial cost, and you have to remember their expansion rate when setting valve lash. With aluminum heads on an iron block, usually you set the valves around .004" tighter than normal when the engine is cold. Recheck things when up to temp to confirm your cold lash setting.

Aluminum heads are less prone to detonation also. Plus they will disipate heat faster than an iron set.

Yea what chevy kid said, delayed detenation can help get you some extra compression