For delta printers, nozzle contact probes are best. This is because it is very hard to get the geometry of a delta so accurate that the effector remains perfectly level as it moves in the XY plane, and any tilting will change the relative height between the nozzle and an offset probe.
Types of nozzle contact probe include:
- Duet3D Smart Effector
- FSRs (force sensitive resistors) or piezos mounted in the bed supports
- Piezo or microswitch in the hot end mounting
Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod
Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].