An experimental Point Spread Function (PSF) is a PSF acquired from measurements on a particular imaging setup.
The best PSF is always the one you measure experimentally, in a sort of calibration of your microscope. It will include all the physical deviations from the ideal model, and your Image Restorations would be much better.
This calibration can be achieved for example by Recording Beads. Because a direct experimental acquisition of a PSF is not always possible, we may need to process the measured beads with a Psf Distiller.
Images affected by severe Spherical Aberration due to a Refractive Index Mismatch are better restored with Huygens Software through the use of depth-dependent PSF's. (That is because Mismatch Distorts Psf). Currently this depth-dependency can be only achieved through Theoretical Psfs, calculating a different PSF for different positions.
See also Theo Vs Exp Psf and Parameter Variation.