ST did an article a few years ago on comparing options SIN-KL.

As I recall it was a wash (time-wise) between taking the bus and flying for the reasons stated in the above post if you started the journey from the CBD and had luggage. Flying came out ahead in terms of time by 30 minutess city center to city center.

Of course, flying may be easier if you live in the East (Pasir Ris) and taking the bus would be easier if you live in West (Boon Lay). Also you don't need to check-in 2 hours ahead if you don't have luggage etc (45 mins before curbside with BP in hand is OK).