Quick comparison table taken from an excellent MSDN article: Comparing the Timer Classes in the .NET Framework Class Library
System.Windows.Forms | System.Timers | System.Threading | |
Timer event runs on what thread? | UI thread | UI or worker thread | Worker thread |
Instances are thread safe? | No | Yes | No |
Familiar/intuitive object model? | Yes | Yes | No |
Requires Windows Forms? | Yes | No | No |
Metronome-quality beat? | No | Yes* | Yes* |
Timer event supports state object? | No | No | Yes |
Initial timer event can be scheduled? | No | No | Yes |
Class supports inheritance? | Yes | Yes | No |