I liked them both, but they each had their own faults. The story in XIII was much better than XIII-2. The story in XIII-2 wasn't well executed, meaning sometimes it gets really convoluted/confusing and other times it turns really simple. I also did not like how the battles were extremely easy until you get to the last area. There, I suddenly met monsters that could 1hko me. I was like wtf?? And the boss battles except the last one were no trouble at all.
At the same time, XIII-2 addressed a lot of the issues with XIII (I shouldn't need to list them all), even small things like adding a physical jump button to make the game feel less linear. And I remember seeing stuff like people complaining about how Square went overboard with the complaints and made the maps too complex instead of too linear. While I can see the reasoning behind these complaints (the map for Academia comes to mind), it wasn't too big of an issue for me because the maps didn't feel as hard to navigate as some were saying. Sure, there are some pointless corridors and sections in the maps, but idk, it made the world feel more real and lifelike? In any case, this wasn't an issue for me either. I also liked messing around with the monster taming system. That was a neat addition.
The one thing I hated the most in XIII-2 was finding the wild artifacts. Omg, it gives absolutely no hints where these are and this became a problem when I needed to find those items for Hope at the end. I also got stuck due to these wild artifacts before this as well. It might keep the game less linear but it's just a tedious and uncreative way to do it.
The main reason I even like FFXIII is most likely Lightning. If it were not for Lightning, I probably would think of FFXIII more negatively lol. I guess XIII-2 helped me like Serah a little more. She looked better in XIII imo.
Conclusion: Both XIII and XIII-2 could have been much better games. So I agree, Square has been slipping up. The setting and characters are there. They could have done so much more with it. We need a new director on FF. That might change things.