The monthly option is better rather paying for semi annually or annually. We do see a large number of emerging companies in web hosting business. Most of them do not even understand what is the value of reliability to the customers.
From my personal experience, I have found that most of the companies are closed down due to any reason. Out of every 200 companies started, only 1 completes its 1 year of customer satisfaction. When you pay semi annually, you are like bound with the company with a virtual contract that you will have to host with them for at least next 6 months and most companies are closed before this time or if they do not, their service is very unreliable, poor uptime which annoys its customers most. So, they loose up the amount they paid for 6 months of service as in most of the cases, the buyers are not entertained any refund since these amounts are usually high.
While paying monthly, its not that much to loose if you do not like the hosting or the company is unreliable. And in such cases, the amount is usually small, so the buyer is refunded unless the owner dont wants to steal your money.
So finally, I would prefer monthly payments rather than paying up a sum of 3/6/12 months.
So, its better for customers to pay few cents more than loosing few dollars and its also better for web hosts to pay few cents more in trnsaction fees rather getting lots of red reputations.