Should one prioritise SRS contributions over the Retirement Sum Topping-Up Scheme (RSTU)?

For this, my guideline is very simple.

If you are young, use CPF RSTU to form a good bond component. Cash can be easily deployed for investment (assuming you have a better investment that nett 4%).

IF you are hitting 55 soon, CPF RSTU is the no brainer, especially if you are hitting FRS. (because the money can be accessed easily after 55 above FRS).

If your tax bracket is 11.5% and above, just maximise both, and seek opinions for investment options.

A little variation off Elijah's reply, with my own perspective now in my third year of pursuing the two.

A) cpf sa earns 4%, no matter what.

B) the money in SRS has to be invested to show better than probably 0.1% interest. And when it comes to investing, then two things kick in - the transaction fees (there are additional transaction fees for srs investments, small but definitely incremental vs investing in cash), and timing / length of investing.

C) putting $$ in either means taking money out from your cash side. The tax relief is real, but it can also be pretty rough if something bad happens (eg retrenchment, or hospital bill) and you can't depend on either to save your most immediate pressing financial concern.

D) on timing of withdrawals, srs (if you have the acct now), you can start withdrawing when you reach 62. For cpf on the other hand, assuming no changes, and you already hit FRS by 55, it is very possible you are accessing the "excess returns" as early as 55.

E) you dont pay tax on the "excess returns" you withdraw from cpf (above your choice of the brs / frs / ers). 50% of the withdrawal from srs will be taxed. I would think that say if you have 1 million in srs, and start withdrawing 100k at age 62, then 50k is tax free, and you would start paying tax on the next 50k on top of your other taxable income.

This is a first world problem though (paying tax is better than being broke).

F) From my female colleagues, I learnt they have not much benefits from doing srs as they generally hit the max 80k relief cap from working mothers relief, cpf relief etc. So they probably only need to consider at most the cpf mstu / rstu.

I have two rules of my own.
1) if I think I cant beat 4% (use last yr srs portfolio return as reference) I prioritize the cpf top up first.

2) to balance on issue C above (远水救不了近火), and I dont think I earn that much, I finally decided that a healthy amount to put into the reliefs was my annual bonus. Coz I would want cash to deal with the rainy days, even though I would end up paying more tax next year. My tax bracket wasn't that high till I should max out both.

I do have a strong feeling that if your incremental tax rate is greater than 15+%, it would probably make sense to max out both (i have not mathematically proven this, its just a hunch).