Go with monthly, to start with. Additionally, if the host offers multiple plans, go with the smallest plan that will run your site - then, after 3 or 6 months, upgrade to the plan you want and prepay for six months or a year. That not only lets you try them out for 3 or 6 months, but if you find that they don't meet your needs, you can move a bit more easily and will potentially have spent less money in that trial period.
Also be sure to back up your site even more frequently during the trial period - again, to make things easier for you if you do decide to move.
