what about the google map app? its free and has turn by turn directions. It can get you to where you need to go, I really dont think you will get an app thats leaps and bounds better. They all operate off the same satelites, so the only changes are going to be ease of use, speed/ refresh rate/ and looks. I think google map gps is pretty good, dial in some settings to improve it.
I dont think voices matter, you wont be able to hear it speak with a helmet. So I am interested in where and how you are mounting your phone, do you have a waterproof case for it? This has always been something that I wanted to do. Are you running a headphone cable into your ears, music, calls, etc?