I got the OBDlink MX (
http://www.scantool.net/obdlink-mx.html) as it provides the fastest read rate and I liked the idea of having closer to real time information in the Torque app. It's also nice as it works on Ford and GM which we also have in the family. This dongle is one of the more expensive going around but is also firmware upgradeable which seems to be quite a rare feature.