From mmo-champion:

Dungeon Difficulty and Rewards

* 10 and 25-player (Normal difficulty) -- Very similar to one another in difficulty; drop the exact same items as each other.
* 10 and 25-player (Heroic difficulty) -- Very similar to one another in difficulty; drop more powerful versions of the normal-difficulty items.

And a player can do either 10 or 25, he cannot do both now and the items will be as the same level.

Reasons to do 25 now?

Please post in the original announcement:
Cataclysm Raid Progression Refinements