Firstly; there really aren't any bokmål speakers, it's a written form. The national broadcaster NRK demands of certain of their announcers and news presenters that they need to adjust their speak to resemble either bokmål or nynorsk.
But then again; a lot of the people living in Eastern Norway around the Oslo fjord, and stretching North up to the great lake of Mjøsa, they write bokmål and they speak with forms and endings more commonly found in bokmål. Oslo being the biggest city, and capital, means this area holds a great percentage of the population.
Of those saying en seng, sengen I think most live in Oslo, Sandvika and down to the Swedish border. However a lot of people will actually say en seng, senga mixing the gender a little (using en for both f/m nouns is actually very common). And some will stay true to form and say ei seng, senga.
What's the most common? My best answer is that it depends on where you are. (There might be some statistics saying what is most common in real numbers, but then I would have to google for a bit.)