Difficult to say with those fish. They are clearly young fish, and the species look very similar. Assimilis tends to have a more wedge shaped spot tapering towards the caudal, whereas filamentosus has a more oval shaped spot and also more forward on the body, but only really by a scale or two, in fact, it is in roughly the same place if you take the taper away.
Both species have red and black markings in the lobes of the caudal fin. assimilis tends to have a black mark to the trailing edge to the lobes, and a red marking crossing the width of the lobe in front of that. In the specimens of filamentosus I kept from juvenile to breeding, the red mark was limited to the outermost few rays of the caudal and did not cross the lobe entirely. That said, the difference is somewhat vague and varies from fish to fish.
Since the caudal markings tend to fade with age, and the spot becomes a less defined roundish blob, the adult fish look increasingly like each other. Personally, I believe the two "species" may be the same fish, just a local difference.
The nomaclature is further confused by the treatment of Systomus assimilis some authorities quote it as synonymised with B. filamentosus, others promote it to B. assimilis.
It's another of those classification questions I'm afraid, sadly, the best answer is probably
