Flora Sky is actually not a spin-off game, rather it is another one of those hacks (though it is really amazing.) However, in your question in which spin-off game to buy, I highly recommend the Mystery Dungeon Series. Among the spin-offs, they have the most beautiful storyline (even better than the core games). Plus, the replayability is the best among all the spin-offs.
among them, however, seeing as you are a beginning player, I highly recommend you buying PMD Red or Blue. To give you some kind of a stepping stone, this game basically teaches you everything you need to know about the Mystery Dungeon world. Once you think you get the hang of it, you can proceed to playing Sky (which is simply Time and Darkness plus some quirks.) There may be new features, but nothing too complex. As I said, everything you need to know is included in PMD Red and Blue.
However, if you prefer getting another game, I suggest you watch out for the upcoming Nobunaga's ambition. The looks of it is promising. It is a totally different spin-off series, which somehow combines the core games and Ranger (I guess). Also, the new Rumble Blast game has awesome graphics, especially for a Pokemon Game, though from what I heard, the story kind of fails.
Anyhow, you'll have fun playing spin-off games. I find them more fun than the core games!