I happen to own both games. True, I've been distracted and not had much time to dedicate to finishing either, but I've seen various Paradox endings and the 'primary' ending that requires no special tricks in XIII-2.
As for XIII-2's plot, I understand it:
Caius had been turned into a l'Cie in order to protect Yeul. Etro, moved by his dedication, freed him from his fate. However, in the process, she granted him eternal life. Over the centuries, he grew to care for Yeul and had to watch her die, be reborn, and die again, always dying young (I think the oldest version seen in the game was only 17 and the youngest was 14). After a LONG time, he grew fed up with Yeul's bad hand and decided to destroy time by killing Etro so she wouldn't have to die over and over again.
Unlike Noel and Serah, he doesn't travel through time. He instead is just living forever and using Yeul's visions of the future to know things as he faced the two protagonists of the game.
I don't know if that is as clear to anyone else as it is to me.
I also enjoy what I've played of both games. While I enjoyed X and X-2's more turn-based style and XII's real-time turn-based hybrid, the speed of XIII and XIII-2's turn-based battles really mean I have to think on my feet. Unlike X, X-2, and XII, you can't just walk around with characters who have the ability to use any magic or ability. In XIII and XIII-2, you can max out each tree, but you can only be specialized in one at a time....ie, you can't cast healing magic while a Commando and you can't deal damage as a Medic. I happen to like that.
As I got XIII-2 as a pre-order gift from a family member, I have that DLC that came with it free. I also have the, so far, one free DLC they've added. And, yes, I am looking at the other DLCs for future purchase. Amoung other things, having XIII Lighting, XIII-2 Lightning, Hope, Sazh, and other XIII characters (major or not (like Lightning's old CO, lol)) as summons that'll fight alongside me in battle if I get their crystal, just adds to the fun, if you ask me.