Matt's got an interesting rundown of the Murtha/Hoyer voting records showing that, generally speaking, Murtha is to Hoyer's right. That said, Murtha still strikes me as the more liberal choice.
In the end, this is less about the Majority Leader than the Speaker. Hoyer represents a more moderate, corporatist stance, and has often whipped in its service. He provided organization and cover for the 73 Democrats -- including him -- who voted for the Bankruptcy Bill. He's got a long-standing personal and political rivalry with Pelosi and will work to establish himself as an independent power center, and will be waiting with a shiv if she slips up. Murtha, who is likely running at Pelosi's request, is her ally first, a centrist second. Given that, this particular contest comes down to how powerful you think Speaker Pelosi should be. In essence, a vote for Murtha is a vote for Pelosi. A vote for Hoyer is a vote against her. And I've few concerns about Pelosi's liberalism.
