How could you feel FFXIII was less linear?
If anything FFXIII-2 made me see what a joke of a game FFXIII really was, a straight line, no extras (something simple like mog abilities made me go back to old maps to discover new secrets), no memorable villain, no replay value, no nothing. I'm sorry but FFXIII just wasn't fun...it felt like an interactive movie, a terrible movie at that. FFXIII-2 just felt fun.
Regarding endings, least XIII-2's ending made sense, FFXIII's didn't even make sense at all, I had to go around forums to read so called "theories" on forums to see how it made sense.
People are liking FFXIII-2 more as it's a far more competent RPG, the only thing FFXIII has over it is graphics pretty much in my opinion.
But I can see if you loved FFXIII then FFXIII-2 might be a disappointment as it is a pretty different game overall.