That unit trust is still pretty expensive, though, at up to 0.725%/year. Ouch. At least it's one of the better unit trusts in terms of expenses, but that's not saying much.
Another big problem with that U.S. S&P 500 unit trust is that the fund managers have to pay the 30% U.S. dividend withholding tax if I'm not mistaken, not the preferential 15% treaty rate available with Irish domiciled funds. So your total returns are going to suffer if I'm right.
I agree with Tangent314. In addition, I recommend SRS only after you've fully exploited more attractive tax reliefs and government grants, such as the Child Development Account, CPF Medisave top-ups, and CPF Special Account top-ups (in that order, I'd say).
Also, not that I believe much in market timing, but by traditional measures of valuation (the Shiller PE ratio, as a notable example) the S&P 500 is pretty richly valued right now. If you believe those traditional measures then the STI is pretty reasonable right now. Maybe the best you can do with SRS funds is a lower cost STI fund, then deploy your external monies more globally and also at lower cost.