I assume you are young. I am retired, so my input may not be helpful. I do a lot of layovers in SIN and just spent several days in KL. IME, SIN is boring. And expensive. Both supposedly have great food, but it's hawker stands and I just can make those work for me.

Both have good public transport, so you can get around.

SIN has good free entertainment on the weekends. Sometimes (I'm not sure when) Marina Bay Sands has free outside movies on the weekends.

I haven't managed to get much "different culture" in either city.

If you want different cultures, great food and cheaper, try Bangkok.

I hope some younger, hipper folks will reply.