The answer to this question basically comes down to a little bit of math...
An annual subscription = $69.99/year
$69.99 * 1 year = $69.99
A monthly subscription = $12.99/month
Let's say you surf only during the spring and summer months (6 months):
$12.99 * 6 months = $77.94
In this scenario, you would save $7.75 and get 6 extra months of Surfline Premium by signing up for an annual subscription.
Why not go for the annual subscription? It might encourage you to throw on a wetsuit (and booties, and gloves, and a hood...) and get in the water during those fall and winter months.
You can start your free trial for a Premium subscription here.