<p>We are using log4j behind a selfmade wrapper. We plan to use much more features of it now. </p>

<p>Should we update to logback ?</p>

<p>(I mean the framework not a facade like SLF4J)</p>

Not exactly answering your question, but if you could move away from your self-made wrapper then there is
Simple Logging Facade for Java (SLF4J)
which Hibernate has now switched to (instead of commons logging).
SLF4J suffers from none of the class loader problems or memory leaks observed with Jakarta Commons Logging (JCL).
SLF4J supports JDK logging, log4j and logback. So then it should be fairly easy to switch from log4j to logback when the time is right.
Edit: Aplogies that I hadn't made myself clear. I was suggesting using SLF4J to isolate yourself from having to make a hard choice between log4j or logback.