Ferret said:
I think I inferred that the Wotc one was the one full of nice new rules (like the PrC paladin, WP/VP etc.) and the other, AFAIK was not so good.
Unearthed Arcana, from Wizards, is indeed a book chock-full of variant rules - it would not be possible to implement them all consistently in a single campaign.

I really like the book, though you should be aware that not everything in it is intended to be balanced with by-the-book D&D. In other words, if you pick it up, don't be freaked out by gestalt classes and the like.
Monte Cook's
Arcana Unearthed, from Malhavoc Press, is (as Eric said) an alternative to the D&D
Player's Handbook. It was motivated by a thought of Monte's - that the gameworld described by implication in the
Player's Handbook is really only one way to "do" fantasy. With that in mind, he created a setting - the Diamond Throne - based on some ideas for ways in which the setting could reflect the rules of the game, and vice versa, such as making the history of the setting important, using PC races of very different sizes (the races presented in the book vary from Tiny to Large size), using a unified but at the same time more complex spell system, and so on.
Basically, if you're looking for something new to throw into a D&D game,
Unearthed Arcana is a good choice. If you're looking for something different to standard D&D but still recognisable as the same game, check out
Arcana Unearthed.
I own both products and plan to use them both in my own game, but then I'm a crazy man.