If you have any experience with Cisco routing and switching then it's hard to go wrong with a Cisco PIX firewall. But if you're not already familiar with Cisco gear then the learning curve may be too steep and you could end up with an insecure config unless you invest in Cisco training. In that case, it may be worth going with the Symantec or Sonicwall solution.
Also, I noticed that someone else mentioned a Smoothwall solution. I wouldn't necessarily rule that out. You could get a standard rack-based server (with about a 1.5 GHZ processor and 512 MB of RAM) from your preferred server vendor and load the commercial version of SmoothWall or the open source product IPCop (which is better than the free version of SmoothWall). These are both robust solutions and are easy to administer. In fact, they are nearly on par with Symantec and Sonicwall. Of course, they aren't as powerful as Cisco PIX.