So far using M1, their 3G speeds have improved considerably as well. Now I can stream YouTube on my iP4S (which only supports HSPA 14.4 Mbps max) with hardly any buffer. Their LTE is pretty okay (I'm getting a fairly consistent 20-30 Mbps on my iPhone 5 around different parts of the country), but not as fast as Singtel albeit still in the "actual" performance range listed on their website.
SingTel is definitely faster (currently I have a line with them, and I used to have 4+internet in the past) but their phone customer service sucks big time. I hardly ever had a pleasurable experience with them over the phone. Online customer service is somewhat decent. Be careful not to get overcharged by SingTel though. SingTel has arguably the fastest 3G/LTE speeds around, but it also varies with location.
Each carrier has its pros and cons. However I would suggest thinking twice before terminating M1 and porting to StarHub. Some of my friends are using StarHub and they always complain about the network. I have not been with Starhub for a long time hence I am in no position to determine if they're good or bad, but I would definitely stick to a consistent carrier.