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.
|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|