As someone who has been an enterprise customer this depends on a lot of factors. First being who your competitors are and how their pricing is structured. At work we are evaluating three products and the one that has a yearly fee will win because the other two charge based on usage. I would definitely tend to start with a yearly model until you have lots of customers.

One thing to consider is, can you put their logo on the front of your website? Some companies I've worked for do not allow this while others will endorse your product. Be sure to think about the overall value of the customer and how you can leverage having them, but don't make your business depend on their contract.