While Im only privy to the way log4net works, an obvious bonus to using that framework is immediate familiarity for those used to using log4j.
Another small benefit is that test driving logging using log4net is extremely simple; loggers implement log4net.ILog. Again Im not familiar with the Microsoft solution, but Im wondering how one would do this without first writing a facade to the System.Diagnostics.Trace class.
With a cursory look at the trace sources documentation, I could not find an equivalent to layouts, and would be interested to know if such an equivalent exists. The PatternLayout is quite handy for formatting log entries with common data like datestamps, thread info, log context etc. Log4net PatternLayout docs: http://logging.apache.org/log4net/release/sdk/log4net.Layout.PatternLayout.html
Additionally, given that writing extensions to a logging framework is probably a classic 'meta-problem', log4net does bring a grand list of pluggable listener equivalents to the table.
List of appenders: http://logging.apache.org/log4net/release/config-examples.html
