VOO has a total (fund) expense ratio of 0.03%, and CSPX has a total expense ratio of 0.07%. VOO has an additional tax drag that's currently equivalent to about 25 basis points, so it's really a ~0.28% fund versus 0.07% for CSPX. CSPX wins this contest. CSPX is also an accumulating fund, meaning the fund manager immediately reinvests all net dividends. That's more efficient than VOO which has to distribute dividends then, maybe, the broker can arrange reinvestment. (Or, if not, you have to.)
...But why are you considering either VOO or CSPX? Wouldn't a global stock index fund make more sense? The popular examples are VWRA, ISAC, and SWRD. Here are the current approximate share prices of these funds:
VWRA: US$101.82
ISAC: US$65.02
SWRD: US$28.09
So if you want the lowest share price then SWRD wins, and it has a 0.12% expense ratio. If you feel you need stocks listed in "emerging market" exchanges — something you didn't think was necessary when considering VOO or CSPX, let's bear in mind — then ISAC wins (0.20% expense ratio). If you feel you need more mid-cap stocks then VWRA wins since it digs a little deeper into smaller stocks than ISAC does.
Then, at a S$800/month pace, if you'd like to reduce commission costs (currency conversion, stock purchase) you can "batch up" your buys into (for example) bimonthly (once every 2 months) buys of S$1,600 each. At a US$28.09 (for example) share price, or even higher, fractional shares aren't going to add much value at all. Because on average you'll only be dragging about US$14 of "excess" cash at the current price of SWRD.