When comparing log4net vs JSNLog, the Slant community recommends log4net for most people. In the question“What are the best .NET logging frameworks?” log4net is ranked 3rd while JSNLog is ranked 4th.
Ranked in these QuestionsQuestion Ranking
Pro Supports Multiple Frameworks
Has all the features you'll most likely need, and is quite configurable. Basically what you'd expect from a very popular and widely-used framework.
Pro Modular and Extendable
Has plugins for most of the things you'll need to get done without having to deal with too much unnecessary functionality.
Pro Widely Used
Using log4net is a safe bet - many other open-source projects use it as well and it'll be just another dependency you don't have to worry about. This also means finding support to fix bugs is a lot easier!
Pro Integrates with your existing server-side logging package
Not a hosted option, so you'll store client-side logs along with your server-side logs.
Pro Stack traces
JSNLog passes exception log messages with its stack trace.
Con Dealing with errors can be a pain
Because of the poor documentation and difficulty of configuration you might end up spending a lot of time just trying to fix things that aren't working.
Con Confusing Documentation
Documentation is quite difficult to navigate through unless you know exactly what you're looking for.
Con No hosted option
You'll have to store client-side logs along with your server-side logs.
Con No reporting capabilities
Your server logging solution should provide that, and JSNlog integrates with Elmah, Log4Net, NLog, Serilog, Common.Logging.