Weight: Aluminum boats are typically lighter when compared with fiberglass models and require less power and fuel to run than that of fiberglass boats of equivalent size. Plus you can get away with a smaller tow vehicle.
Maintenance: Fiberglass is more susceptible to fading from ultra-violent light, and if you don't maintain it, the boat will look chalky and faded in a few seasons. With aluminum you can typically just rinse down the boat.
Durability: Bang into a dock or a rock in a fiberglass boat and you'll wind up with a deep scratch or chunk out of the gelcoat. A scratch in aluminum is often easier to fix.
Tradition: In fresh water, fiberglass is associated with run-abouts and cruisers, but the hard-core anglers seem to all use aluminum. Why swim against the current?
Resale: It's related to tradition, but if you plan on upgrading in a few years, buy a boat that people in the area would want to buy from you one day. If everyone's using aluminum, they're not going to buy your used glass hull, no matter how awesome it is.