I have played them all but it's been years so is there a difference or are they all just sequels?
The main difference is gameplay. Budokai plays more like a 2D fighter (with 3D sidestepping and dashing), and the Tenkaichi series has full 3D movement and dashing. The Budokai has generally better story cinematics than Tenkaichi, and Tenkaichi is known for its huge roster.
There's quite a bit more differences, but those are basically the major ones. I can list more if that doesn't jog your memory.
The original name for the Tenkaichi titles was "Sparking!" but "Budokai Tenkaichi" was used for western audiences to connect the Sparking! series to Budokai's fame. The Budokai series was made by the company DIMPS, and the Tenkaichi series was made by Spike.