BlackIce Defender is not really a "firewall!" It's a N.I.D...
ZoneAlarm has the ability to hide ones footprint on the net, this makes it harder for someone to shoot you if they can't see you
BlackIce has never been responsible for locking or crashing a computer it has been installed on (generally speaking), and uses very little resources
ZoneAlarm has locked and/or crashed a computer it has been installed on (5/7), and uses from 9% to 15% of Windows resources
BlackIce can't block an out-going packet
ZoneAlarm can block out going packets
These 2 programs might be marketed to the same group of users, but they are essentially different type of programs.
If I was on a dial-up (56K), I would only use BlackIce Defender.
If I could only use 1 program (BI or Z/A) for a DSL or Cable connection, I would choose Z/A.
For my needs, I use both and find that they compliment each other, I feel naked any other way
------(Forgot to add)---------
Since the "heading" is firewall, lets not forget the "hardware" firewall approach. Currently I am only using the software version, but I think the ideal firewall would be:
Hardware firewall (Linksys, Netgear, ect...)
-- and --
BlackIce Defender
To me, that would be a perfect firewall for under $150!