FFX is much more linear than the FFs you've already played. Following the one and only road for the whole game was what ruined it for me. However, the aspect of voices for the characters and Blitzball sorta made up for it. If you're one of those people that like to "master" games and do everything you can possibly do, expect to spend a lot more time doing so with FFX.

FFIX has a world map~ It's a lot more entertaining IMO. Since you've only played FFVII and FFVIII so far, then it seems that you're way overdue for some moogle, black mage, princess and summoner action. If you liked Triple Triad in FFVIII, you'll probably like Tetra Master too, just not as much. If you hated Triple Triad in FFVIII, you'll probably love Tetra Master because it's much easier. Unlike FFX, the characters here are quirky and funny instead of lame and stupid.

Now, I hope you're not one of those people that instantly dislike FFIX claiming that it's a kiddy game because the main character has a tail.

Bottom line. Pick FFIX.
PS: hey, that rhymes
PPS: hey! so does that~