The difference is that a symmetrical, or semi-symmetrical airfoil will not provide as much lift as a flat-bottom wing. Therefore, a flat-bottom wing will produce more lift at slower speeds, allowing you to fly more slowly (which is often a good thing for novices). The disadvantage of a flat-bottom wing is that when trying to perform maneuvers (like a split "S") as you generate speed in the down-line, the plane will try to "Balloon" when you level out because the excess speed is creating too much lift.

The Dihedral is for stability. Think about the kites you used to fly as a kid. There was always a string tied between the ends of the cross member to "bow" it toward the back. This, essentially, is dihedral If you look at the first pic below, you'll see how that bow on a kite works the same way that dihedral does, and vice versa.

When one side dips, the air can hit it more squarely, whereas it will deflect off of the other side. This creates a self-righting tendency.

By the same token, if you look at pic 2 you will see how dihedral will make it difficult to fly upside-down. Now, the self-righting tendency is not only gone, but you have the opposite effect. As one wing dips, it LOSES it's ability to catch air, while the other wing GAINS ability, so the wing wants to return to it's upright position.

To answer your next question, I would say that if you started on a semi-symmetrical wing, you have skipped the first step, and now you are passed that. Move on to a more advanced plane or you will probably get bored quickly.

You sound like a great candidate for a 4-Star 40.