Well I know you are trying to beat all the FFs, excluding FF XI, but out of the two you have listed as to get first I would choose FF X. Both are linear games though Final Fantasy X offers more freedom; one could say that Final Fantasy XIII is not linear but controlling, as the game controls so many facets of itself leaving little for the player till deep into the game. Final Fantasy X does have more mini-games and definitely one of them is hard. Final Fantasy XIII on the otherhand lacks in mini-games in comparison to FF X.

I like Final Fantasy X's music more than Final Fantasy XIII's, but that is more your call.

The voice-acting was better in Final Fantasy XIII however I liked the characters (the whole cast) in Final Fantasy X better. I also prefer Final Fantasy X's story vastly over Final Fantasy XIII; I feel that there is a lot that needs to be explained in Final Fantasy XIII. Final Fantasy XIII has a great premise but execution needed improvement. This game was in need of somebody like Larry David (Seinfeld writer) who was known for keeping meticulous records on the show's characters so every little detail would fit.

From a money perspective. If you don't have a lot of money or are wanting to not spend a lot purchase Final Fantasy X as it is cheaper than Final Fantasy XIII that is still recently new.