I still think XIII was a better game as well, of course. Be prepared for 20 hour long story games from S-E from here on in most likely. That one year dev cycle for FF games just turns me off.
In retrospect, it's like this for me:
Final Fantasy XIII-2 is a better 'packaged' game. It has a lot of stuff I expect in an RPG. Essentially, it's more 'fun' than Final Fantasy XIII. It also has better characters in Noel and Caius. Also the drop rate in this game is awesome. Switch leader speeds up battles.
Final Fantasy XIII is probably the better game when it comes to the scenario, graphics (FFXIII-2 kind of makes me feel nauseous at times with regards to the framerate), and difficulty. I am not the biggest fan of CSB (Command Synergy Battle -- what the XIII battle system is called). Compared to dresspheres, I think it's a little inadequate.
However, what made CSB shine so much was difficult battles. Final Fantasy XIII had the right amount of difficulty in its late-postgame and the Bart battles. It emphasized the importance of Jammer, Enhancer, and Defender to the FF audience--and audience of players who--in my experience--normally did not emphasize buffing, debuffing, and tanking in previous numbered FF installments. The late-postgame battles against the Undying and the Guis made players think about which Optimae to use and equip, what exactly you should equip to get 5-stars for rarer drops, character animation times, efficiency, ability synthesis, watch your stats so that they don't get too high, etc.
In this manner, Final Fantasy XIII's approach to the battle system is superior to Final Fantasy XIII-2's approach. Final Fantasy XIII-2 is too easy (and I'm in the minority who thought FFXIII was also, for the most part, easy outside of Guis casting Ultima consecutively). Final Fantasy XIII had the issue of not supporting CSB adequately for most of the game. Final Fantasy XIII-2 has the issue of not supporting CSB adequately almost at all. Jammer and Enhancer are rendered almost unnecessary outside of the occasional use (and this is because Enhancer was severely gimped in FFXIII-2 to my dismay), while in FFXIII, they seemed to be quite essential in Chapters 9-13 for making battles easier and end faster.
Separating the SEN skills in FFXIII-2 was a little weird to me, too. Making Serah and Noel both tanking-type of SENs struck me as a little odd, especially since Noel lacked Steelguard and Vendetta, and Serah lacked Mediguard (which seems to be essential if you're a tank-type of SEN, imo) and Entrench. The best way to do this was to make Noel a tank-type SEN, and Serah an evade-type SEN (ex: like Lightning and Hope). At least FFXIII's attack distributions seem to make sense. Here, they kind of do and they kind of don't.
Basically, I don't really like the ease of FFXIII-2 because battles ended up being somewhat boring. Even the 'harder' encounters seem easy, and if you're using monsters, battles become a joke. Again, as emphasized in this thread, I generally play games differently from a lot of people, so if I find it easy, some people may not find it easy. So I guess it seems like it's a ymmv type of thing, but from glancing at this thread briefly, it seems as though it is easier than FFXIII outside of the Last Episode (and part of what makes the difficulty ramp up there is because you're so busy using unstrategic and inefficient Optima to get through battles quickly in other episodes), and this is not a good thing for CSB at all.
To conclude, Final Fantasy XIII-2 complements Final Fantasy XIII well. Whether that is a good thing or not is up to you. I don't think either game out of the two stand out at all. Like with FFXIII, I feel a little indifferent towards FFXIII-2. I think both games are middle-of-the-road.