Well, if you have never bought into either product or type, you've got to first ask yourself if either of the two available options under POSB invest-saver fundamentally appeal to you or not. Are stocks through the STI ETF, or bonds through the Singapore Bond index fund something that appeal to you, and whether you have an appetite for either type of product?
If you do already like stocks or bonds, then by all means go for it for this is an easy way to invest in them and of course comes with the added advantage of the 0.3% interest on savings in the bank account.
I would caution against investing in them for the additional 0.3% interest, although it is of course possible as others have mentioned to invest in either and then withdraw each month. To me that is entirely pointless and something I would never do.