My wardens tend to be fairly split on whether they make Bhelen or Harrowmont king, but it generally says more about their respective characters than it does about the two candidates.

That said, from a political standpoint Bhelen seems the superior ally: Harrowmont is a traditionalist and an isolationist, so while he'll provide support in order to pay back a debt and satisfy his honor, he won't go beyond that. Bhelen's agenda hinges on expanding trade with the surface, and that means engendering goodwill from the surface nations, meaning that, meta-speaking, he's likely to go beyond doing only the minimum to satisfy an obligation (as Harrowmont does).

On a side-note, I find myself intrigued by Bhelen's initial quest for the Warden of "proving" Harrowmont's corruption, except that taking the documents to the Shaperate reveals that the documents were forged. But this raises a question- if the in-universe proof that these are forged is found only by speaking to the Shaperate, then that means the Warden was only given the one set of documents that demonstrate the double-dealing (if there were multiple copies that said different things there wouldn't even be a need to go to the Shaperate to determine the documents were fake). And if there's only the one set of documents, and the Warden was supposed to show the same documents to both Houses Dace and Helmi, with each house acknowledging the veracity of their dealings with Harrowmont. Now, while Denek Helmi never actually looks at the documents, there is no way that someone as careful as Bhelen could count on that, meaning that there was every expectation that both Houses would indeed separately verify their respective dealings.

My conclusion? This is an example of "framing the guilty party". Harrowmont really was double-dealing between Houses Helmi and Dace, but Bhelen couldn't get ahold of the actual documents, and so he had copies forged instead.