Hah, that's an interesting question. I've never even had the IDEA of comparing those two games. And I really had some hard time deciding which was better.
As I am not able to say which one is better, here's the detailed version:
Overall enjoyment of the environment & towns
I chose: both
First off: IX. I enjoyed the game, I really liked it. (I wouldn't name myself Freya if I didn't like it... xD") It wasn't that difficult, but it wasn't easy either. Sometimes I got stuck and then had to find my way out, which was quite amusing. I remember that I played nearly every day to finish the game, I always waited for evening to come so that I could play again. :3 The environment... well, I liked the world of IX. It wasn't as sad as the world of VI, it wasn't as strange as VII and VIII, it was just a normal fantasy world. And it was looking good. ^^ The towns were also nice, I especially liked (guess what xD") Burmecia, I could've stayed in this town forever. It was a really sad place to be (as it was destroyed and all), but the rain was... well, how to put it? It just had an atmosphere I liked. So many memories, so much destruction... it seemed so... beautiful, in a way. It's hard to explain. ^^" But I also liked the other towns. x3"
Then: X. It was kind of the first FF for me and it was pretty damn much fun. I played about 4 hours the first day I got the game. I think that says it all. xD" After a while, it got more difficult, but in the end... it was quite great. As for the environment & towns: I loved the atmosphere, I loved Spira. <3 It was a great world, with so many great places to go. There was no place (except for Besaid) I didn't like. I especially liked Zanarkand. Like Burmecia, it was just a.. melancholic place? Dunno, I really can't describe what I felt. ^^"
So, both games had an awesome atmosphere and were fun. (Short version of a long text .-.")
Battle System
I chose: X
Pretty easy to explain. I like to think about what I'm doing, I don't like to rush through a fight. I tend to just press X like a maniac all the time when I've no time to choose what to do next. So FFX had a better battle system in my opinion. I really thought about what I was doing, I could see when the enemy would strike again... quite nice. x3 FFX was one of the only FF's in which I really used magic - just because I could think about using it. xD"
Characters
I chose: Both, again
That's a hard one. Both games had characters I didn't like and characters I loved. I... don't have to mention my favorite character in IX, do I? It's Freya, of course. I loved her story and all about her. The fact that she's a kind of rat, that she lost her beloved one, that she was some kind of fighter, always strong, etc. etc. There is so much more about her I liked. <3 There were also a few characters I didn't like that much, like Garnet. Somehow I really disliked her, she was useless most of the time. ^^" (Especially the few hours in which she couldn't concentrate anymore... I completely switched to Eiko then xD") But the stories of all the characters were so deep, it was just awesome how much there was about them...
X... you may throw stones at me now... had Auron. Auron > everything. Auron's the best character ever made. Auron is awesomeł. I think I don't have to say much about it, I posted a novel about why I like Auron before on this forum. I have a wallpaper of Auron... it's been there for 1 ˝ years now. xD" I'm just a fangirl, sorry. XD
Side Quests & Minigames
I chose: X
So this is what I experienced: I'm bad at finding side quests and minigames. I didn't find much in IX and somehow rushed into the final boss. On the other hand, in X I found much to do. Currently I've spent 100 hours on X and still haven't finished everything. It's just awesome how long you can play this game - and it's still fun.
Some time... I have to play IX again and try to find the side quests. ^^"
(Btw. I liked the card game of IX more than I liked Blitzball ^^")
Boss Battles
I chose: neither one~
I'll keep it short this time... I didn't experience that the boss battles were more fun or longer or more difficult in one of the games. So both are equal in this point. I had much fun beating the enemies in both games, but it wasn't THAT great.
Storyline
I chose: IX
There was no other way... X had a sweet story, I really liked it, all about Tidus and the Fayth and Sin and such. But it lacked... philosophy, in a way. I didn't really think about what the game could tell me. (Except for: Enjoy the time you have with your loved ones xD") So... no really interesting story for me.
IX had an awesome story. At first you wouldn't believe it was so.. deep. The real story starts when you first see the Black Mages and find out how they were made and stuff. Then the characters all began wondering why they were alive, when they'd die, what was the meaning of life... and I also thought about it. Especially at the end you got much to think about. I won't spoil anything, but I wouldn't have expected it to be like that. .,.
Yet, in X you could pretty much imagine what would happen. There were only few twists in the story.
Weapon & Character upgrade system
I chose: IX, again
Don't get me wrong, I liked the Sphere Grid. But I didn't have the feeling I ever improved. I got more HP and my attack and defense and stuff rose, but... I didn't see any progress. I didn't know which level I was. So IX was way better. Besides, I liked the fact that you could learn abilities by equipping your weapon and armor. That was a nice idea. x3
X lacked such creative ideas somehow. You just had to buy the weapon and then build it yourself... that was a bit... well, for me, bad. I normally wasted everything and then didn't have any items left to make a better weapon. xD"
So, now I counted what I chose. 4:4
I can't say which one really is better. I love both of them. >//<
@Locke:
One last comment, on these threads: You should add "Music" to the categories. I think it's also quite important. ^^
Bookmarks