According to MSDN "in Microsoft SharePoint Foundation 2010 the preferred method of writing to the Trace Logs is to use the SPDiagnosticsServiceBase class" (http://msdn.microsoft.com/en-us/library/ff512746.aspx).
MSDN also provides some guidance on the trace and event log severity levels to use (http://msdn.microsoft.com/en-us/library/ff604025.aspx), however the WriteEvent() and WriteTrace() methods use slightly different enums; the diagnostics logging configuration in Central Administration is slightly different again, and then you have a third set of values accessed by the PowerShell command Get-SPLogEvent.
The table below shows the mapping of levels from these different sources.
Despite the complicated mapping, in general I think things go in the right direction with events writing to the event log and trace log at the same time, and having a high trace level. The distinction between event logging and trace information is also good, with independently set thresholds.
| EventSeverity | EventLogEntryType | TraceSeverity | ULSTraceLevel | ULSLogFileProcessor .TraceLevel |
| None = 0 | None = 0 | 0 (None) | Unassigned = 0 | |
| ErrorServiceUnavailable = 10 | Error | 1 | Critical = 1 (or ErrorCritical) | |
| ErrorSecurityBreach = 20 | ||||
| ErrorCritical = 30 | ||||
| Error = 40 | ||||
| Exception = 4 | ||||
| Assert = 6 | ||||
| Warning = 50 | Warning | 8 | Warning = 8 | |
| FailureAudit = 60 | ||||
| Unexpected = 10 | Unexpected = 10 | Unexpected = 10 | ||
| Monitorable = 15 | Monitorable = 15 | Monitorable = 15 | ||
| SuccessAudit = 70 | Information | 18 | Information = 18 | |
| Information = 80 | ||||
| Success = 90 | ||||
| Verbose = 100 | ||||
| High = 20 | High = 20 | High = 20 | ||
| Medium = 50 | Medium = 50 | Medium = 50 | ||
| Verbose = 100 | Verbose = 100 | Verbose = 100 | ||
| VerboseEx = 200 | VerboseEx = 200 | VerboseEx = 200 |