The 3.5/Pathfinder Handbook

[pres_man]

To the paladin issue, comparing the two:

Code of Conduct
A paladin must be of lawful good alignment and loses all class abilities if she ever willingly commits an evil act.

Additionally, a paladin’s code requires that she respect legitimate authority, act with honor (not lying, not cheating, not using poison, and so forth), help those in need (provided they do not use the help for evil or chaotic ends), and punish those who harm or threaten innocents.

Ex-Paladins
A paladin who ceases to be lawful good, who willfully commits an evil act, or who grossly violates the code of conduct loses all paladin spells and abilities (including the service of the paladin’s mount, but not weapon, armor, and shield proficiencies). She may not progress any farther in levels as a paladin. She regains her abilities and advancement potential if she atones for her violations (see the atonement spell description), as appropriate.

Code of Conduct
A paladin must be of lawful good alignment and loses all class features except proficiencies if she ever willingly commits an evil act.

Additionally, a paladin's code requires that she respect legitimate authority, act with honor (not lying, not cheating, not using poison, and so forth), help those in need (provided they do not use the help for evil or chaotic ends), and punish those who harm or threaten innocents.

Ex-Paladins
A paladin who ceases to be lawful good, who willfully commits an evil act, or who violates the code of conduct loses all paladin spells and class features (including the service of the paladin's mount, but not weapon, armor, and shield proficiencies). She may not progress any further in levels as a paladin. She regains her abilities and advancement potential if she atones for her violations (see atonement), as appropriate.

The only real difference I see is the removal of "grossly". So, yeah, PF's paladin's rules are stricter than 3.5's. Technically a 3.5 paladin can violate his code, as long as it is not "grossly", while a PF paladin can never violate his code, even in the cases where two different aspects of his code conflict, without falling.

Also someone mentioned that a paladin only falls if they commit an evil act. Sorry, the text does not support that in either edition.