A few things....
1. First, the cast iron top is deeper, which gives you more space between the front of the saw and the blade, as well as the back and the blade. Think of it as trying to balance a board on sawhorses 1 foot apart, or 8 feet, which is more stable?
2. The cast iron top is wider, allowing you to easily cut wider things.
3. The cast iron top is heavier, a heavier saw is more stable, and easier to cut on without worrying about it tipping.
4. The motors......amperage is unfortunetly not all created equal. Ever looked at a shop vac that claims to be 6 hp......its not....The motor in the bosch saw is whats called a direct drive universal motor. The motor in the ridgid saw is a induction style motor, when it says 13 amps, and 1.5 hp....it means it. Much better torque and cutting power.
5. Rewiring your saw to 220 will not give it more power, it cuts the amperage in half, thus allowing you to have lighter wiring running to it. Now many that have rewired to 220 do report it feels like more power, and thats likely to the idea that a saw on 220 will spin up faster, so it seems to recover faster when bogged down.
6. One other option....the craftsman 21833 has a larger motor than the ridgid 4512, it goes to 15 amps from 13, and 1.75hp from 1.5. there are a few other differences but all in all...its a almost identical saw with some upgrades.