I like to play Tales of Symphonia. It's an AWESOME game. I'm easily on like my 7th playthrough just for Grade/Gald farming.
I own Tales of Symphonia: Dawn of a New World (Knight of Ratatosk in USA) and, while it's definitely not as good as Tales of Symphonia, it does flesh out the story a little. Also, yay it has the Tales of Phantasia overworld theme.
I then play Tales of Phantasia to finish off that universe. I have never finished Tales of Phantasia though, 'cuz at one point my HDD died and I lost the savefile and don't wanna be bothered to re-play it, argh. I bought the GBA version but it doesn't seem as good as the SNES version. Although I like the game and the system, I wished the game used brighter colors for certain things. Seems like it has off-colors and too many shades of brown at times.
And yes, Tales of Phantasia is a distant sequel to the Symphonia titles. You can see similarities even in some stages, like the Ymir forest and stuff. I wonder if I should try playing the PSOne version as it looks a bit nicer. Anyone know?