Peter
The Downside of a Single Mass fly wheel is as described above, however you should also consider the upside:-
Much improoved Engine response,both up the revs and back down. This enables the engine to "free up" giving better performance and quicker gear changes.!!!
However I must concede that these benifits are probably more usefull for track use where the additional transmission noise within the car may not be an issue.
If your car is purley for road use, where comfort is primary, then I would probably recommend you stay with your "Dual Mass",which will undoubtably be more compliant.
Regards
John