VERY roughly, look at your marginal tax bracket (e.g. 7%) then divide by 2, to reflect the ~2 percentage point gap between these two choices. That’ll tell you very roughly how many years it’ll take for the SA top up to pull ahead and leave the SSB in SRS in the dust.
Let's improve on this very rough calculation. Let's suppose your marginal income is all within the 15%
tax bracket and you have an additional $10,000 on an after tax basis to save. You're trying to decide between your Special Account and a Singapore Savings Bond in your SRS Account. (Let's assume here you cannot make a MediSave Account top up because it would bust the CPF Annual Limit, so that's off the table.)
I'm also going to assume that you're willing to take the future income tax savings and apply it to your immediate SRS Account deposit, and let's also assume a 2.0% interest rate on your Singapore Savings Bonds, net of costs ($2 transaction fees) and with reinvested coupons.
OK, here's what the Special Account deposit would look like over the next 10 years:
Year 0: $10,000.00
Year 1: $10,400.00
Year 2: $10,816.00
Year 3: $11,248.64
Year 4: $11,698.59
Year 5: $12,166.53
Year 6: $12,653.19
Year 7: $13,159.32
Year 8: $13,685.69
Year 9: $14,233.19
Year 10: $14,802.44
And here's what the SSB in SRS Account deposit would look like over the same 10 years:
Year 0: $11,500.00
Year 1: $11,730.00
Year 2: $11,964.60
Year 3: $12,203.89
Year 4: $12,447.97
Year 5: $12,696.93
Year 6: $12,950.87
Year 7: $13,209.89
Year 8: $13,474.08
Year 9: $13,743.56
Year 10: $14,018.44
The "back of the envelope," very rough calculation suggests you take the marginal tax rate (15%) and divide by the yield gap (2.0) to arrive at the number of years to reach "break even" between the two scenarios. So in this case the rough calculation suggests that the Special Account will start to pull ahead (and stay ahead) of the SSB in SRS after 7.5 years. And the more careful calculation backs up the rough calculation: the crossover point is at 7.X years. (A little closer to 7 than to 8, due to compounding effects.)
Sure, tax savings are nice, but as this example shows they're not all important. And really we've been a little too generous to the SSB in SRS choice, because we've pulled the future tax savings to the present. When you make a decision to get tax relief, the tax savings actually occurs sometime next year. If we're even more careful and fair then we'd put that $1,500 (15%) of tax savings into a separate SRS deposit the following year, but this is close enough.
Also, it's not a given that your SRS withdrawals will be tax free. They may not be. In this comparison we've implicitly assumed they will be.