Is it wise to use platform Interactive brokers to buy and keep S&p 500 instead of a normal brokerage in Singapore?

Interactive Broker is very cheap, with the lowest fee in the world (probably) and the widest range of products. They are known as mass-market stock brokers. If you are doing only $100 a month, it can still be cheaper in IB despite the $10 monthly fee by IB, if your stock is bought in overseas currency. This is because IB's forex rate is hugely with no hidden spreads in some banks and brokers. You're saving quite a big difference on the forex.

1. Commission is lowest. (some products is negative commission, means you get paid to trade). Most of my commission is $1 or $2 for 100 stocks. And I get paid commission for trading in non popular stocks as I adding liquidity to them.
2. Variety is the most wide in this broker.
3. Forex rate is one of the most competitive (a lot of brokers have hidden fees due to the spread)
4. They have a local Citibank account, so you can FAST transfer SGD, and the SGD reflected in your account within minutes. Then go platform and exchange to USD or whatever you want on your own.
5. $10 minimum fee, so if you only trade $4 this monthy, they will charge extra $6 to round it up to $10.

5 is a bad point, but 1-4 makes it worthwhile.

In short, yes. All traditional brokerage in Singapore will not be able to beat IB in in terms of cost and market accessibilities

Assuming that you buy S&P 500 ETFs domiciled in the US, commision is at US 30 cents for every 100 shares excluding taxes and exchange-related fees. Total comms usually come at under 40 US cents. Note that IB has a monthly charge of US10 if your account is less than 100kUSD