Whether or not Solaris makes a better firewall is debatable. Solaris firewalls are probably more common in the corporate world. Being "better" would really depend on the environment, the hardware, budget, uptime requirements, and admin skills.
For simple firewalling needs you might look into SunScreen on Sun's website. Another free alternative is Darren Reed's
. Of course there are other options such as Checkpoint or Raptor, but I don't believe that you can get a free/demo version from them.