i'm a registered user of both (kerio and zonealarm pro

) so here are the verdicts...
zonealarm is easier to use (for beginners), free version is good enough for most applications, it is slightly less stable but (here it comes) severely sucks when using LARGE numbers of ports / HUGE amounts of traffic (think anything p2p, after a few days you have to reboot the machine if, for example, you're running something like emule 24/7)
i found it easier to get zonealarm to work with games, and managing security levels is a bit easier as well
kerio may be a little less friendly (imho ymmv etc.) but it works fine as a gateway firewall, and doesn't give any troubles with p2p stuff
kerio did f* up with the last update though, classifying all windows (os, filesharing, etc.) traffic as 'medium severity intursions' means the latest version is pretty much useless for anyone running a homenetwork, must be a bug (i hope) and i expect them to fix it soon... duh
another thing i didn't like about kerio is the way they handle vpn's... on the dutch kpn adsl network ppoe is used by default, declaring all that traffic as 'secure' if you use the default ruleset (this does not apply to those not using ppoe / pptp)
both packages are supposed to support ics stuff only in their pro versions, well, this is and is not true, it just depends on what you're doing... i got both of them working in their free versions on an ics machine
if you're using something like nat32plus, you can use the standard versions, as they don't check traffic 'going through', remember these are 'personal' firewalls, and do little to protect machines behind the gateway
just need a simple client firewall? go for zonealarm... need it for a small homenetwork on the gateway machine? go for kerio (perhaps registered)
you can always mix and match
