SYMPHONIASYMPHONIASYMPHONIA.
OK, here's it laid out for you:
-ToS has a lot more replay value, a less obnoxious main character (but a more obnoxious heroine), but is more cliched. If cliche bugs you don't play it; if not it's a great game.
UPS OF TOS:
-Affection/different endings
-a hundred gazillion plot twists
-a kewl main character (in my opinion.)
-a hundred gazillion side-quests
-just a really fun and enjoyable game.
DOWNS OF TOS
-cliche
-no 'free run' (see Abyss ups)
-Colette is an annoying blond clumsy goody-two-shoes...
-...that's it.
Now Abyss is similar to ToS but without the affection stuff, and obnoxious main character.
UPS OF ABYSS:
-Free run!! This lets you run freely around the battlefield and not just in one direction. (hard to explain...)
-yet another creative and awesome plot
-mostly the same stuff as ToS, minus the affection
DOWNS OF ABYSS:
-No affection/different endings, thusly less replay value
-DIE LUKE DIE YOU EMO WHINY BRAT
-That's about it
...Just personal preference, I guess. You should try out them both. :]
BTW, there is one thing both games have that totally ROOOCKKK...
You can get multiple costumes for everyone. :]