Longer stakes pay better because longer stakes generate more shares as bonuses for the staker. Roughly you gain 20% bonus shares per year you are staked, up to roughly 5 years max.
In the Axion contract, there are 5 Big Pay Days(BPD) and the first is day 351 after launch. A Big Pay Day is a bonus for stakers and an incentive to have active stakes for longer periods of time. The tokens rewarded at BPD comes from unclaimed Freclaim Axion tokens.
To be eligible for BPD payout your stake has to be active for 350 days or more. It does not matter if you place your stake 5 days prior to a BPD, as long as the total length of the stake is 350 days or more.
1st BPD pays 10% of total unclaimed Axion tokens
2nd BPD pays 15% of total unclaimed Axion tokens
3rd BPD 20%
4th BPD 25%
5th BPD 30%. Year 5 is also the last year with BPD.
Of course, there is a downside to make long stakes, and that is that no one knows what the future will hold. So it's important that you take some time and think out strategies that suit you.
Here is a graph showing you the power of Axion compared to HEX regarding the Return of Investment(ROI) taken from the Axion Whitepaper. The Y-axis shows the ROI and X-axis the percentage of the total supply that is staked.