Don't use cable discs on the rear, they feel awful - spongy and with complete lack of feedback due to the long cable and all it's twists and turns. The only proper solution is a hydro. I've ran both a BB Avid and a Hope, and there's no comparison between them on the rear wheel. However, as a front brake, I'd say the Avid is better. In terms of power, a 180mm Hope will outperform an HS33 even with the best of pads and a grind. Haven't had experience with smaller rotors but a 160mm is probably good enough. And don't worry about hitting the disc, it's not as easy as it may seem - even with a 200mm rotor.