Macro lenses???????photography?

What is the difference between a f2.8 macro lens and a f4 or 5 macro lens? I want to take close-up pictures of flowers and I want a macro lens. Sometimes the flowers are under the forest canopy so they can be in shady environments. I called a guy about a lens today and he said he had a f2.8 for $680 and an f4 or 5 for $250. I would go for the cheaper lens but if it won't take the photos properly then I will buy a more expensive lens. Let me know?