For most cases, the newest option, the QXT/QNT high performance digital position transmitter is the best choice. It features a non contact mag res sensor and combines high precision feedback with easy set-up and configuration via push buttons. This one is designed to be very robust and able to perform well in very harsh environments, like high vibration or wet / humid applications.
The QX5/QN5 features a potentiometer based 4-20mA position transmitter. It is sufficient for most applications where high precision is not needed and the device is not exposed to high vibration. In high vibration or high cycle applications standard potentiometers will typically function, but may have a shortened life. This is caused by the wiper wearing a groove in the resistive element. This will cause the reading to jump as it climbs over the edge of the groove.
QX7/QN7 is also potentiometer based, but utilizes a high performance, precision, MIL-spec potentiometer. It features a hardened co-molded resistance element and precision stainless steel ball bearings. This version provides precise, reliable feedback and withstands high cycle and high vibration applications.
So if you need precise feedback or have high vibration application select either of the high performance options, otherwise the standard version should perform well in most other applications. One more comment, If SIL Certificate is needed for your 4-20mA position transmitter, then your options are limited to Q_5, and Q_7.