My company has corporate plans with both Singtel and Starhub and they are about the same in pricing and features. Chose Starhub as they had a better Blackberry plan so all my voice plans are with them too.
Reception & island coverage wise --> Singtel
Value for money & good customer service --> M1
I had some problems with M1 reception last week. For 3 odd hours, I could not receive/send sms and make/receive calls. I was in Tampines (not some gawd-forsaken place) too. My colleagues who are on M1 did not have this problem. I could not decipher whether it was a problem with my phone or the service provider. But I realized I had alot of other problems with this network when I was using other phones.