It depends in part what you want to do with it. The DL discs tends to be expensive and unless you need the space (over around 4.5Gb), then you should save the money and only get the single layer. The RW are re-writable, so you can use them over again instead of writing one time. The question of either "+" or "-" seems to be more complicated, but I have seen that both tend to work. Make sure that you get the correct speed for the burner (or not burn at a speed exceeding what the disc is rated at).
The main thing is to get a quality brand, which is not just a known brand. I have not kept up on who actually makes the discs vs. the name on them, but thoguht I would reply to part of your question.