American or British voiceover?

As long as its clear and easy to understand, you cant go wrong either way.

do both and setup a simple split test with Google website optmizer to see which one converts higher.

Until you have hard data for yourself, all one can really do is speculate

It would really depend on the subject matter that you are trying to discuss.

If you're talking about a subject that is casual or doesn't have huge ramifications. An American voice actor would probably be the way to go. The reason I say that is because you're trying to market to people who would like to hear the information from someone like them. I think it's called imprinting.

if you're trying to position your product or service or whatever your advertising about, to an American audience and its casual, then the audience will be accustomed to hearing from a white male with a neutral accent in the 30ish age range.

If on the other hand you're trying to advertise for something that's a bit more empirical or is on the important side. A British voice actor would be a good way to go The reason for this is because typically Americans think of the British accent is being "classy" and it can be a bit incongruent to use it for product that doesn't require that intrinsic class to be implied.

Hope that helps out my friend.

I'd go for american if it's for US audience

I think if the target audience is American, then go for Amercian unless your video is about something British related.

You need to test this with Google optimiser. What's the point guessing?

Is the person speaking on the podcast American or British?
The intro should be the same or it will sound odd in my opinion!