Technically, Starsiege was actually the THIRD game in the "Starsiege" series.
Don't forget the first two Earthsiege games, which occured in time around 400 or 500 years or 1000 years previously to the events in Starsiege. While the game was fun for a while, it was most definetly not as good as Tribes.
I tested both games when they were in their closed betas, and everyone always had more fun playin Tribes. Besides that, they're two different genres.