The longer/more expensive subs are better value.

You always get the same benefits, including the same number of crowns per month and the same discount on crown store items, no matter which one you pick, but subscribing for longer is slightly cheaper per month.

On the subscription page it shows you the price per month, as well as the total price (you pay the whole lot in one go), so you can see how much cheaper it is. For example here's the UK prices (on PC if that matters):