Best way to initiate a pay per month membership scheme
Try by doing this

Create a Membership type ABC with monthly rolling with auto renew required option ticked.

Create a price set with price field as select or radio with two options
2.a ABC membership type (monthly) - monthly amount - number of terms set to 1
2.b ABC membership type (yearly) - yearly amount - number of terms set to 12

Include the price set in membership signup form


Or

Create a Membership type ABC with monthly rolling with auto renew required option ticked.
Create a Membership type ABC-yearly with yearly rolling with auto renew required option ticked.
Include both the membership type in membership page

(Second method will work, but not sure about first method)