as far as i can tell, it seems to depend on which ff game is your first as to which half of the series that you like more. if 1, 2, 3, 4, 5, 6, 9, 10, and 12 are your first and/or favorite, you will like the more medieval fantasy games more and if 7, 8, 13, and 15 are your first exposure to the series and your favorite(s), you will like the science fiction themed games more.
13 has a lot of problems with just the first game alone; but it probably comes off better if it's one of the first that you played. ff13 would have been like the 15th or so in the series that i played so it just falls flat for me. there's quite a bit of stuff that would have worked and made it a better game and story but they went with what they did.
this is also seemingly a case where the person in charge liked the game, ideas and characters so much that it was forced to be completed, regardless of how much it would sell.