I think neither method is perfect. So you better apply all of them, and calculate an average.

Cookies are not very good because:

• not everybody turns on them
• some people are using more machine, and more browser, they will be counted multiple

unique IDs from access logs are also not the easy way to the eternity, why:

• there are gateways and proxies presenting unknown number of users
• robots used to request pages

...and many other pitfalls, so i recommend the averaging method, or trust it on webalizer and other professional (and free:) stuff.

-- tune "turn off the light, take a deep breath, and relax..."