I think you will find the answer you are looking for very much depends upon variables unique to your product and space: your customer base, the product itself, its price, etc.
The conventional wisdom from an accounting stand point is that a yearly subscription is more advantageous than a monthly subscription. That is because the cash for the subscription is captured up front, but the revenue is "recognized" over the course of the year. But I think the benefit of this is only relevant if it is compatible with your accounting model.
What I think you have grapple with is the price sensitivity of your customers. Some customers, purely from a psychological standpoint will find it much easier to pay $10 per month, rather than $100 per year, even though they would save money doing so -- simply because $10 is easier number to wrap their head around than $100.
Given that my answer to your question all hinges upon the cop-out response of "well, it depends," then here is what I suggest to hopefully get you pointed in the right direction:
Conduct thorough A/B testing.
What is A/B testing? For those that don't know, A/B testing is when you segment users into two groups. Group A gets presented with one option and group B with another. Then you compare your results to see which group tracked in a more desirable direction with regards to behavior, conversion or acquisition. This approach requires a scientific mind - not literally, but procedurally. Go into every A/B test with a hypothesis. Then be sure to segment your groups so that the conclusions you draw can be clearly correlated to a single variable (or two). Consider also to maintain a "control group" in which no variables change as a baseline. Then look at your results, see if your hypothesis was supported, and if not, try to ascertain why. Was the experiment flawed? Was there an error in execution? Is the hypothesis flat out wrong? etc.
So if your goal is to acquire more customers, and your question is "will the choice of a yearly subscription at a lower cost help me acquire more customers?" Then I think your A/B test, or experiment, will be pretty straight forward. Give some users the choice, and others the same choice they have today. Or perhaps your question is, "at what price will a yearly subscription become more appealing than a higher priced monthly alternative?" And so then it is all about trying out different price points and seeing how that impacts acquisition.
