The general recommendation by antifreeze manufacturers is to have a 50/50 antifreeze and water mix. Increasing the ratio of the antifreeze to water such as a 70/30 mix will lower the freezing point of the mix down to minus 72 degrees Fahrenheit, but it will also lessen its ability to cool the engine. A higher water to antifreeze ratio increases the ability of the mix to cool the engine, but the mix will not do very well in cold weather.
A higher antifreeze to water ratio may be beneficial in cold weather because of its lower freezing point ratio, while a lower antifreeze to water mixture will have improved cooling effect in hot weather. Keep in mind, however, that most modern car engines have sensors that give engine temperature readings that are calibrated to the 50/50 mix, so a different antifreeze to ratio mix might result in false readings.