Assuming FRS is not hit and no cash top ups have been performed to either. For both the purpose of tax savings and saving for retirement, should you do cash top up to CPF SA or SRS account first? Why?

Do $1 top-up to SRS now (to lock in retirement age) or end of year when banks have promo.

Depends on where you going to invest your funds later.

CPF
4% guaranteed
Narrower scope of investment
Cannot withdraw until 55

SRS
Wider selection of investment tools available
Able to withdraw with 5% penalty and add into tax for the year

Some pro and cons I can think of.