Where the sites do differ is in the depth and quality of their coverage. On Snopes.com the Mikkelsons go to extraordinary lengths to address the finer details of each text, supplying critical analysis as well as background and contextual information. Most importantly, they cite sources.
Not to disparage TruthorFiction.com owner Rich Buhler, who does maintain an up-to-date and generally trustworthy resource, but by comparison his analyses tend to be perfunctory, and his sourcing minimal at best.
Snopes.com boasts a 15-year record of providing accurate, dependable information and analysis, and in that time has earned the confidence of the media, government agencies, the business community, and the general public alike.
Given all of the above, Snopes is surely the preferable resource.
UPDATE: The Bud Gregg incident
A subsequent variant of this rumor purports to describe a verified instance of political bias on the part of Snopes.com:
Excerpt from forwarded email contributed by Paul S., Oct. 29, 2008:
A few months ago, when my State Farm agent Bud Gregg in Mandeville hoisted a political sign referencing Barack Obama and made a big splash across the internet, 'supposedly' the Mikkelson's claim to have researched this issue before posting their findings on snopes.com. In their statement they claimed the corporate office of State Farm pressured Gregg into taking down the sign, when in fact nothing of the sort 'ever' took place.
I personally contacted David Mikkelson (and he replied back to me) thinking he would want to get to the bottom of this and I gave him Bud Gregg's contact phone numbers - and Bud was going to give him phone numbers to the big exec's at State Farm in Illinois who would have been willing to speak with him about it. He never called Bud. In fact, I learned from Bud Gregg no one from snopes.com ever contacted anyone with State Farm. Yet, snopes.com issued a statement as the 'final factual word' on the issue as if they did all their homework and got to the bottom of things - not!
As claimed directly above, the Snopes.com page in question concerns a political (anti-Obama) sign erected by Mandeville, Louisiana State Farm Insurance agent Bud Gregg. Snopes.com indeed states that Mr. Gregg was asked by State Farm's corporate office to remove the sign. But whereas the above text claims that "nothing of the sort ever took place," State Farm has confirmed in writing that "Management requested the sign be removed as soon as its presence became known."