We (FRC1640) use BI Technologies Magnepot Hall Effect Potentiometer Part # 6127V1A360L.5FS. Cost is reasonable ($12.32 each). These sensors have performed well for us since we adopted them in 2012. They are analog devices. We calibrate them mechanically and identically on all of our swerve modules.
see:
http://www.bitechnologies.com/pdfs/6120.pdf