There can't be any valid comparison between L4 and minix because the IPC mechanisms on Minix are obscenely crippled. They managed to wrestle defeat out of the jaws of a triumphant victory... =((((
The problem with L4 on the other hand is that it doesn't have an operating system wrapped around it in order to make it usable... I have a number of earth-shaking projects that I want to develop but can't for lack of a reasonably good IPC mechanism and no way to develop for L4 for reason of it's obfuscated system libraries and no development environment. =(((
Got a counterexample of an installable and usable L4 development system?