The difference between the two is in the name: opticalRendu has a fiber optic SFP port while the ultraRendu has an RJ45 ethernet port. Copper ethernet and fiber optic both offer galvanic isolation, making them an ideal connection between digital components. Fiber optic is also immune to EMI & RFI–a characteristic not found over ethernet. This is why many report a lower floor noise after implementing fiber optic.
Unless your server has an SFP port, you will need to use a fiber media converter to convert ethernet to fiber optic and then into the opticalRendu. This adds another device, another power supply, another cable.
@sns recently switched from a SOtM sMS-200 to the opticaRendu. Perhaps he can share his impressions.