Monster Hunter Freedom Unite is, ultimately, Monster Hunter Freedom 2 with several new features. As someone who has played both without getting too far in either (as of yet), I was surprised to discover just how much the little changes have already improved my experience with the game. I will say up front that I am glad I purchased this game despite already owning Monster Hunter Freedom 2.
Take, for example, the new "Buy to Box" option available at pretty much all vendors. It's perfect for when one is buying an item they don't intend to take into the field, such as Worms for use as fertilizer at Pokke Farm. Similarly, when planting seeds or fertilizing rows at Pokke Farm, players can use materials currently in their storage box without having to withdraw them first. They're minor features, and they make play that much more enjoyable.
One of the more prominent features, and one that becomes available surprisingly quickly, are Felyne companions. They follow you into battle, their cute level is off the charts, and they're really handy for both grabbing items you can't/won't/don't have time for (they don't need a pick axe to mine or a bug net to catch bugs) and for combat purposes. Depending on their personality, they may avoid fights, or they may charge headlong with a bomb ready to throw.
In summary, this game is well worth the purchase. It has all the quality of Monster Hunter Freedom 2 pumped up with a host of new features ranging from small but significant improvements to awesome new features that can alter how you play the game. Enjoy!