The guaranteed sum portion is giving you an interest rate of 1.6%, still better than putting your money in the bank. But this doesn't tell the full story, you need to look at your non-guaranteed sum too, which should be in your BI. While that sum is non-guranteed, most likely the amount you actually get will be nearer to the guaranteed + non-guranteed than just the guaranteed sum, unless the markets are going to be really bad for the next 15 years.
BTIR would generally be better, but not everyone can learn how to invest correctly, and it's only recently that it has been made easier for the masses with online brokerages being easily accessible. Insurance company's saving plans is probably the most accessible, marketed and easiest way to get one to start saving, and for sure the returns are way better than just leaving the money in most bank deposit accounts or even fixed deposit.
The problem of course is that the insurance companies structure the plans such that they suck most of the commission out of you in the first 2-3 years. By the time you know about BTIR, it becomes more worth it to see the plan through to maturity.
If you cancel your plan, you can simply just invest $200 in ES3 and $100 in A35 (or vice versa if you are are more risk averse) every month through POSB Invest Saver. Instead of A35, you can buy SSBs every few months (since minimum is $500 for SSBs) if you can discipline yourself to do that.