That is with hindsight. Without it (when you first get to Orzammar and first time hear about Harrowmont and Bhelen), the issue is between a man who would resort to any means necessary (murder included) to ensure he gets the throne and someone who wouldn't go that far. I don't know about you, but knowing only that (and not the exact future consequences of the choice), to me it's clear what to choose - not the madman (or "maddwarf", if you wish).
Except that, unless you're a Dwarven Noble, you have no way of knowing how true the rumors against Bhelen are. You only hear secondhand sources from individuals that range from unreliable t obiased, and both Harrowmont and Bhelen are doing everything they can to slander each other. But even in all the various comments and rumors you'll hear from various people in Orzammar and the two criers in the Diamond Quarter, there are a few truths that become apparent. One is that Bhelen is aware that a Blight is underway and is planning action against the Darkspawn, while Harrowmont is hesitant to get anything underway. Two, Harrowmont, ever the staunch traditionalist, and his ilk regard the Casteless as scum, while Bhelen and his ilk appear to be stirring up sympathy for them. Three, Harrowmont's supporters are leery about opening up further relations to the surface, while Bhelen wishes to increase trade. Never mind that learning about Dwarven politics reveals that actions like Bhelen's aren't anything unusual, which leave one wondering just how honest Harrowmont is being.
And it's incredibly evident from Harrowmont's first quest just how utterly useless he is as a leader; he can't even get his own men to fight for him in some worthless Proving match. If Zevran's in your party when you speak to Harrowmont's second, http://www.youtube.com/watch?v=5hUoPQwnY-E.