I just evaluated log4net and NLog for usage in a bigger project. Both have a similar interface and are highly configurable.
NLog seems to be better maintained: An incompatibility of log4net with .Net4 remained unresolved in log4net for quite a long time. Nlog comes with some more 'bells and whistles' like a NuGet Package and a Xml Schema for Visual Studio for editing config files.
In the end we decided for log4net because we measured a much better performance for log4net: A simple test, writing 10000 log messages to a file and to a network log viewer (Log2Console) showed a ten times better performance of log4net! We did no tuning in the config file, file and notwork logger were used with a minimal configuration. You should verify this for yourself with a typical logging setup of your project.