Jesus is entirely big picture: spelling out the basics of the field and trying to answer the question of who Jesus was. Five Gospels does very little of either and spends the vast majority (~90%) of it's pages on the job of color coding Jesus sayings. Each passage is accompanied by a brief explanation of why things were colored the way they were.
The books also reach widely different conclusions. Ehrman argues something that many scholars have believed for the last century: that Jesus thought the world was going to end within his lifetime. The Jesus Seminar instead argues that when Jesus spoke about the Kingdom of Heaven (or, as they translate the phrase, "God's Imperial Rule") was something which Jesus saw as "already present and of more elusive nature."
In terms of general plan, Ehrman's book is far better. The Jesus Seminar states broad conclusions with little support. No wonder they got torn apart by conservatives: they never defend their major conclusions. Take the example of the authorship of the gospels. The Jesus Seminar simply asserts that they were not written by the usually assigned authors. They mention that the texts were originally anonymous, but unlike Ehrman, they fail to explain why scholars believe this. They give only a couple of sentences to Papias' testimony, and hardly anything to looking at Irenaeus and John's internal evidence. Ehrman looks at all these issues and explains why they don't establish traditional authorships. He also has reasonably detailed examinations of such issues as extra-biblical sources for Jesus. His book is, in short, a fine source for getting your New Testament scholarship 101. The Five Gospels gives the lay reader next to no help along those lines.
Seeing The Five Gospels I think marks the end of my ability to take the "great teacher" view of Jesus even half seriously.. On p.11, they say that "God's imperial rule is the theme of Jesus' teaching." Then, when discussing what this means, they say the following:
Scholars are agreed that Jesus spoke frequently about God's imperial rule, or, in traditional language, about the kingdom of God. Does this phrase refer to God's direct intervention in the future, something connected with the end of the world and the last judgement, or did Jeus emply the phrase to indicate something already present and of more elusive nature?...In short: Jesus started his career in association with an apocalypticist, his first followers were apocalypticists, our only sources for his life frequently portray him as an apocalypticist... but he wasn't one? (The Seminar tries to argue that Jesus' ideas "was almost entirely lost on his followers.") Only the most blatant wishful thinking, the strongest will to believe that Jesus was a great teacher, could possibly lead them to such a conclusion. They unintentionally make Ehrman's view look quite credible.
The texts [i.e. sayings of Jesus] cited in this cameo essay can be used to support either view. One thing is clear: John the Baptist and the early Christian community espoused the first view: they believed the age was about to come to an abrupty end. (p. 137)
Another serious problem with The Five Gospels is that the explanations of why sayings were colored this or that way quickly become tedious. I tried reading through, and off the top of my head now I cannot remember reading anything of use. What, one wonders, is the point?
I can only think of one answer: the Jesus Seminar, after realizing that the New Testament cannot be totally trusted, wanted to produce a new inerrant text, which liberal churches could use, confident that the genuine sayings of the great wise man could be sorted out from later, ignorable attributions. This is a quite hopeless task. Our sources just aren't good enough to pinpoint the authenticity of everything that Jesus said.
It is a real shame that Ehrman's book never caught the public eye as the Jesus Seminar did. Public knowledge of Biblical scholarship might have been greatly improved for it.
(Side note: does anyone know how to become a referrer for Amazon.com? I'm thinking of putting links to the site with referral codes in my book reviews, but I have no idea how to do that.)