I believe that a Paladin/Antipaladin should not be a base class. They are to specific to be the base class. I understand alignment requirements for classes (such as any Lawful, any non-Good, etc.), but having a very specific alignment requirement makes them sound like a Prestige Class to me.
I believe the base class should be something more like a templar (a Full BAB, Good Will Save, Crusader-like Class). A templar would not have casting, but they would have many unique combat abilities to make up for it (think Crusader's Steely Resolve).
They would then get the ability to gain a Prestige Class as a Paladin, Anti-Paladin, or Holy Vindicator, etc. This allows specific holy warriors for all the alignments. The entry would be easier then a normal prestige class (something like Level 5), and would be much like the Prestige Paladin from Unearthed Arcana.
I would be fine with this as long as the Paladin was mechanically different from these other classes and got to keep toys that no other class got.
That was actually my logic behind making them Prestige Classes. It would now justify having a rather specialized class about being Lawful and Good. I beleive base classes should have some breathing room so that many different types of personalities can flourish without being punished. A Prestige Class can be more strict as a character has chosen to go to the next level on their own accord.
The Antipaladin would most likely be similar to the Blackguard, but with some newer features.