Personally, I'd use the mixture. Anti-freeze not only lowers the freezing point of water, but raises it's boiling point reducing the possibility of overheating. All of the better anti-freezes also contain rust and corrosion inhibitors and water pump lubrication components. Some even claim to help seal small leaks.
A word of warning. Most anti-freezes are made from ethylene glycol. Ethylene glycol is not only extremely poisonous but sweet tasting. Even small amounts taken internally can cause serious illness or death. If you're concerned about spilled or leaking antifreeze, purchase a propylene glycol-based antifreeze or coolant, such as Sierra or Prestone LowTox. While no antifreeze product is completely harmless, using propylene glycol antifreeze does provide an added margin of safety in the event of accidental ingestion.
