even since i never used renderpal i am tempted to recommend it over muster.
i use muster since version 1.0 and i got more and more disappointed over various bugs and the creators not really great way of customer support and communication.
i had some longer chats with the guys doing renderpal and i made some suggestions what else they could implement - and 2 weeks later they had it all in!
i would really test drive them both - since they have about the same features and pricing in by now. if they are close to equal in serving your needs i think going for renderpal would be the better idea imho.
the virtual vertex guys tend to bring out their new versions with bugs and questionable default settings (a bit like alias themselves). and after installing new versions to dozens of new computers i can get quite pissed when i see that some features simply do not work and i have to reinstall an old version.
nevertheless: i dont wanna flame over muster: it is running with some effort and i rendered some million pics with it already. it is just that you have to know the limitations well to have it working.