I played both (symphonia1 first) and I really think that you should play Tales of Symphonia 1 first. You meet most of the characters in TOS2 and you won't know who they are if you skip TOS1. There are also A LOT plot references to the original TOS, and plot spoilers (especially the the key surprises and the ending of TOS1)
the characters make many references to TOS1, you'll be totally lost and won't be able to understand what they're talking about.
also, i think that TOS2 is quite an updrage from TOS1 in many ways:
-LOTS more voice acting
-much better animation
-better battle system with a few issues fixed
-monsters and all the strategy that comes with them
so if you play TOS2 and then decide to go back and play TOS1 you'll feel like you're playing an inferior version.
Besides all that, TOS1 is a wonderful game I'd recomend to any RPG fan, especially if you like fantasy elements, mythology, angels, gods...etc