Does it make sense for me to open a POSB SAYE account for the additional 2% interest rates, or should I buy SSBs instead?

Personally, I discontinued SAYE since StanChart JumpStart (up to 20k) provides an alternative with better liquidity. The issue for me is the bonus interest is only credited on 13th and 25th month from 1st deposit so during which the account is basically earning minimal interest. And if you accidentally transfer out/withdraw money from the SAYE account, no bonus interest will be given. In essence the account works more like a time deposit rather than a high interest account.

Considering you're using SAYE which needs a 'SAL'/'PAY' component, you can use Seedly's savings account calculator to find a savings account that suits you the most. There are those e.g UOB ONE/OCBC 360/... which requires salary crediting and meeting some transaction requirements which you can use to hit more than 2.5%.