You only need what you intend to watch. So it will boil down to the content you signed up for. The links below will tell you what you get with both memberships.
Also be aware that each will renew at the prices you just paid, month by month.
Unless you want both to continue monthly, go to My Account (above)/Manage Account (the tiny option)/My Memberships, and Cancel what you don’t want, going forward. (You still get all of the month you have paid for - Cancel just means that it does not renew).
With Cancel, ensure you complete all four steps, for each Membership.
Your account will continue nonetheless, for if you want to take out more Memberships in the future. Now operate a ‘Hotel California’ policy here 😛
Please also note that if you want to watch any Now/Sky movies over Christmas, you will also need a Cinema Membership; the things above apply to that also.