As you can probably see by now, there really isn't an answer to your question that's useful. Every flier likes a different feel and a different set of flying characteristics, and the prop has a huge impact on both. Broadly, the difference between the 11x6 and 10x7 will be a trade off between acceleration and vetical performance for top end speed. If you go down to an 11x5 or a 10x6 you'll notice quicker acceleration in your engine which may or may not give you more speed or vertical depending on the airframe. What's important is that the plane flies like you want it to. For beginners, I suggest going with more acceleration since takeoffs can be hairy and recovering from stalls is faster with a big prop.