I've used both Sygate Pro and Kerio 4 and, to be quite honest (IMO), I found both to be excellent.....SYGATE wins on features (which for novice users may be overwhelming or even go un-used), KERIO on cost.
The one problem I did find with SYGATE is that sometimes it 'half' crashes (a bug?)...the icon disappears from the tool tray (yet its still running in Task Manager).
Kerio has NEVER let me down in anyway and I have my machines connected for months at a time without any rebooting.
So, if I had to choose I would say KERIO is the most stable, and its price cannot be beaten.
Remember tho' that ANY firewall is only as good as the person who sets it up..they don't perform magic.
HTH
By the way, the extra features of KERIO that cancel after 30 days are not required, use a proxy like the PROXIMITRON or PRIVOXY, you can also use the TEA-TIMER service in SPYBOT and run and install SPYWAREBLASTER...for even more protection (all free).