I have been both MOL and Singnet ADSL user...both are equally good and fast...one thing is Singnet normally have higher upload speed....so if you like to upload stuffs...Singnet shuold have the advantage.
To me, my main selection critiria.
1. Price - If in the same speed, whoever can give me better deal win
2. Coaxial vs twisted pair - MOL win in term of cabling realiability because it was better technology compaired to the twisted pair copper cable of Singtel. So if you are staying in old housing block, chances of having cabling reliability problem is high. Just one week ago my office ADSL went down after upgrading from 2MB to 6MB. Eventually we found out one of the jumper cable in the riser oxidized because old building are quite antique. The problem only arose when the line migrated to ADSL2, so it seems to be more sensitive towards medium condition.
3. Cabling point - Some home or room have retricted phone point or SCV point, so it might affect the positioning of the modem.