Both this oil painting of Pericles and the following Roman bust of Pericles are so similar to the man depicted in your work that I'm fairly confident it is indeed Pericles. I'm not an expert in Greek & Roman art though, so take it as you will.

The portrait of Themistocles in the vatican, while close, has a fuller beard. This is also reflected in the full size statue of Themistocles in Piraeus. I can't rule it out completely (The collar may signify something I'm missing) but it seems Pericles is the one being shown. What did the antique dealer say when you bought the picture, and if you have the title see if it in any way indicates the sea, a ship, or a naval battle as that would point to Themistocles.