Well both plans do mention "up to" 2 years of coverage and both are valid for up to 2 incidents. But I'm still not finding anything that prevents the cancellation of the monthly plan after 1 or 2 incidents are used, even if it's shortly after the plan started.
As far as the math goes, it only makes sense to me to go with the full payment plan if you're betting on having one or both incidents remaining to cover you between months 21 - 24, which is a relatively short amount of time. Unless there's some other advantage to the full payment plan that we're missing.