If you want more features then Rh10 has to be better. If you don't want the new features and already have Rh9, that could be argued to be better. Your question is too loose to answer properly.
See the RoboHelp Tour on my site for what is new in each version.
See www.grainge.org for RoboHelp and Authoring tips
Hi Sowptik and welcome to the RH forums.
This is an impossible question to answer without knowing more about your specific requirements. You may want to check out the following links to help you decide: