It is clear that the SSPV, just like the Orthodox are in schism, but both have valid orders (I do believe however some have questioned this as I think they’re Thuc-line, which some sedes and others have questioned), and hence have valid Masses (or Divine Liturgies in the case of the Eastern Orthodox). Since this is the case, I would go to the SSPV Mass, as I am a Catholic of the Latin Rite, not the Eastern Rite, and that I prefer the TLM over a Divine Liturgy. Because the SSPV doesn’t require that their members believe in sedevacantism (just like the CMRI), I would just ignore any sedevacantist talk during the sermon (if in fact that is talked about during the sermon) and focus on the Mass. I would however not go to any of the classes offered by them. I know why I don’t believe in sedevacantism, but others might not, and I wouldn’t want anyone being influenced by sedevacantist falsities.
As I stated earlier, I do believe some have questioned the validity of the ordinations of some of the SSPV priests (obviously not the ones ordained by Archbishop Lefebvre but the others). I would research this further, and if in fact they are valid I would go to the SSPV Mass. If I believed they’re not valid, I would probably go to the Orthodox Divine Liturgy.