If you design your RSG app for HD and manifest ui_resolutions=hd, that should work fine in SD/FHD as well. If you design your RSG app for FHD and manifest ui_resolutions=fhd, that should work fine in SD/HD as well. If you forget to declare ui_resolutions, the app will have issues in one of the modes and be bounced from certification.
Advantages of designing for HD is smaller image file sizes (2x as a rule of thumb) and hence 1/2 the traffic - and that it fits better the current UI resolution usage (most players do UI in HD, that's not the same as setting display to FHD). Advantage of designing for FHD is future-proofing your app: if you won't re-do the assets during the next year or two - go for FHD outright.