If all else were equal and you chose A, would you support older, major versions, e.g., would you provide a 2.0.1 and a 3.0.1 together to address a problem that exists in both? If not, then the primary difference between the two is that A gives you opaque, flexible control over when to raise funds from you existing user base and how much you want to raise while B is transparent and fixed.
Option A allows you flexibility to adjust the cost and time points of your iron triangle by allowing you to raise revenue to cover a larger upgrade or faster delivery of upgrades (or to do the reverse without angering your customers.) It also offers your customers more control and predictability because they only pay for what is available now, not for a promise of things to come, and only when they want.
Option B depends somewhat on your reputation, some customers will be less likely to buy a year if they can't tell how many upgrades they'll get, or if they'll get any at all. So some people will be more willing to buy B if they perceive they're buying a service that improves but doesn't change drastically instead of an application that should be growing significantly (Evernote v. Photoshop.)
Note that licensing approach is related to but not causal to the version distribution within your customer base. As Skype just reminded us all very loudly, even free doesn't mean people will stay current. Many other companies (e.g. Microsoft, Mozilla) have discovered for us that even aggressive auto-updating systems leave some users out of date.
Option A gives both you and the customer more control and predicability. It also slightly relieves the pressure on you to deliver major versions faster, perhaps with lower quality or less rationality (e.g., customer perceives there's not enough improvement to warrant the 3 to 4 you shipped largely to placate your annual customers, lowering your marketing v. honesty score.)
The best place to get the answer is from your customer base. Talk to them them directly and research the vendors of other products they tend to buy. Many of those vendors won't be direct competitors and may be willing to talk about what they've learned about pricing models in your market.
If you can't decide, you could allow customers to pick during the first year, giving you an A/B test of sorts.