Here is my understanding as a fellow patient, but you should definitely confirm it with a surgeon.
Brief history: The first resurfacings back in the 70s were metal on plastic (similar to the Synova, but with inferior early generation plastics), and they were a disaster with very high failure rates, so everyone quit doing them. The consensus was that the large joint surfaces in a resurfacing, as compared to the small ball THRs that were being done at that time, caused excessive plastic wear debris. Only a few were metal on metal back then, and they did very well. Interestingly, Dr Pritchett who recently developed the Synova was highly involved with resurfacing back in those early days.
Then in the 90s, the metal on metal approach was developed, and has been doing very well with the right manufacturers and the right surgeons. MoM has done particularly well in the larger sizes, and sometimes not quite as well in the smaller sizes. In an attempt to solve some of the metal debris problems from edge loading in the smaller sizes, Dr Pritchett developed the Synova as a metal on plastic alternative to the the MoM. Since the plastic being used currently has far better wear characteristics than that used in the 70s, it is expected to do much better than it did back then. However, there is still some concern that the metal on plastic might lead to a lot of plastic wear debris in the larger sizes, so I believe that the Synova is mostly being recommended for the smaller sizes that may not do as well with MoM. I believe that Dr Pritchett who developed the Synova is still recommending MoM for the larger sizes, I believe because the MoM will have less wear problems in those larger sizes than metal on plastic.
Bottom line, if you are in the mid to large size range, it is likely that everyone would recommend that you go with metal on metal if you want a resurfacing. If you are very small boned, then the Synova is an alternative to consider. Since the Synova does not have as long a track record as the MoM options, there is some uncertainty there, so it might be worth considering the option of MoM with one of the top surgeons like Dr Gross, who has developed techniques that show good success with MoM even in the smaller sizes. If you have a known issue with metal, Synova might be the only resurfacing option in any size.
Hope this helps.