It depends on what suits your needs at the time of booking. Saver fares are always cheaper than Flex fares. However, a Flex fare always earns more Skywards and Tier Miles, so can be useful if you are trying to boost your Miles to reach the next tier or save up for a reward. Flex fares can also have other benefits such as no eligibility or purchase restrictions. In addition, fewer Skywards Miles are required to upgrade a Flex ticket to a higher cabin class.