Not a scientific, but a practical performance comparison between the two Hexanon lenses.
Conclusion: The optical difference is not as big as the price difference, both lenses have their strenghts:
While the f1.8UC can focus very close and gives a bright viewfinder picture, the f3.5 is optical near equal while beeing more compact.
Optical: The f1.8UC gives much better contrast and snappier colours, but corner sharpness is not so good wide open.
Mechanical: The f1.8UC is more bulky and heavy, while the f3.5 is quite compact.
Equipment: Konica T3n, Fuji Reala 100, Manfrotto 190 Tripod, Mirror and Aperture prefire from the Self timer, cable switch.