The only bias Snopes has is toward ad revenue. I stopped visiting that site when I would get hit with at least 3 pop-ups, pop-unders and redirects on every page.
But on-topic, why do you care so much about proving this to your friends? They obviously won't believe it no matter who it comes from.
A). To cut down on the spread of chain-mail, as I hope that people would get in the habit of fact checking before spreading lies, rumors, and money-making schemes. Remember that many of these chain-mails are simply vehicles for the originator to collect valid e-mail addresses, to spam the collected recipients or to spread spyware. You would understand if you worked in IT.
Most of the chain-mail I get is from friends and co-workers.
Not to mention that this forum is in general committed to stopping the spread of lies, rumors and money-making schemes.
B). To save time in conversations where these e-mails are used as "facts" to support one contention or the other. This becomes annoying to the extreme when one hears the same misconceptions repeated ad infinitum.
C). My friends and I argue about various things. Its fun. If somebody brings up what they think is a good point, and I disagree with their conclusion, I attempt to refute it.
D). Snopes is a handy and time-saving tool to accomplish all of the above. If it's biased, I want to know that: I will stop relying on it. If it's generally fair-minded, I want to be able to prove that so that I can continue to use it.
My friends are reasonably intelligent people. If a conservative friend were shown a video with, say, Bill O'Reilly using Snopes as a reference source, I would have ammunition to refute some anti-Snopes rebuttals.