In sheer terms of which was the better crafted game, FFXII wins hands down. The decision's a little tougher for me personally. As such all following statements are 100% opinion.
Both games weak points were their casts, for me. 50% of XIII's was just annoying, whereas XII's was just almost wholly forgettable. Similarly both casts have someone that saves it from being a complete wash - Balthier and Sazh, respectively (and they are two very difference characters, just as a point of note). I really like Lightning too and have no qualms with Fang...so it comes down to whether having more memorable characters is worth the price of really annoying ones. Beats me.
I like the worlds in both games. FFXII gets a bonus because I love FFTactics and therefore have a predisposition towards Ivalice (and I kinda wsh there was jsut a bit more connecting it to Tactics), but it's breadth, variety, and sheer size is remarkable. It's the only Final Fantasy that gives the feeling that a game like Oblivion or Skyrim would it terms of area to explore. For all the complaining about XIII's lack of exploration, I and XII are pretty much the ONLY FFs where I felt like I was actually exploring. And Rabanastre is one of the most living, breathing virtual cities I've ever seen in a video game, it's up there with Midgar. (And it has great music.)
XIII's world it also pretty cool and mysterious. The idea of this pantheon of (demi?)gods that each serves a specific function is a really cool one. Titan manages the food chain, Phoenix provides power to cocoon, Carbuncle the food, another tills the land, so on and so forth. Pretty neat. They squandered it, but nonetheless I was interested.
The gameplay is where XIII gets it's large boost for me. I had an absolute blast with the battle system and happily did every single Cie'th mission just for more of it. It's fast, and despite what people say, it is very strategic, it's just a wildly different strategy from the rest of the series. Traditionally, FF is a very lower-level, micromanaging strategy and I love that don't get me wrong. But I enjoyed XIII's higher-level 'big picture' strategy every bit as much. The fact that you're selecting auto-battle isn't the point (although the game doesn't do itself any favors by literally making that all you do for like 2 hours in the beginning), it's almost more akin to a real time strategy, selecting the right units rather than the right moves.
FFXII's was very traditional, and it was fine, I have no problems with it, it just took awhile to adjust to the fact that moving around doesn't actually have any bearing on anything, it's still the same numbers deciding whether you get hit or not. What really gives XIII the edge over XII is XII's incessant grindiness. You have to grind all the time in XII, and I was okay with it once or twice, but by the time I was about to go to Arcadia and once again every single random enemy was wiping the floor with me I was getting really tired of it. I never fled from battles, that should be enough.
XIII, on the other hand, not only doesn't require grinding while still having some tough-ass battles, but if you are stuck, grinding is NEVER the answer. I tried grinding once or twice for a hard boss, it didn't help at all. The problem was always the strategy with which I was approaching the battle (being too aggressive, not being aggressive enough, etc.) And I LOVE that.
Both games (once 13 opens up) do themselves a favor by having areas that are totally accessible but WAY out of your league, and I like that too. It gives the world some life, rather than seeming like it was built for you and you alone.
Again, XII is unquestionably the more complete game, but they both have their fair-share of questionable decisions. XIII's are more broad in scope, the lack of towns, the 20-hour "tutorial" phase before you can actually get to the meat of the gameplay, the lack of sidequests. Why is the upgrade system COMPLETELY blind unless you use a guide?
XII's are more niggling. With the aforementioned grindiness, WHY oh why do the Marks not give you experience? I mean seriously, that would at least be something to strive for and hide the fact that you're "grinding." But nope, they give you jack shit in terms of experience. How does that even make sense? Were my characters not there? How did they not learn how to become better fighters by that battle?
And the sidequests, while plentiful, don't really have any variety to them, it's all more combat. There's no REAL diversion from the gameplay, but really I don't think they've done that very well since VII, so maybe it's better that they don't try.
And why is the quest to get the best weapon in the game so stupid. Not only is it arbitrary as hell, it encourages you to NOT do what the game is at its best for - exploration. Like XIII's upgrading, it requires a guide (or being a shitty gamer and therefore not opening chests - of course who could blame you what with all the out of the way chests containing 1 gil), and that's dumb.
Whoa...that was longer than I thought. Because of the gameplay, I might have to give it to XIII personally. But I really did enjoy both games and I think I'm a happier person for it, so screw the haters