Awkward one depending where the op wants to go, Sandhills is best on the Northern Line, probably Birkenhead Central/North & or Park on the Wirral that's if you want out of tunnel shot, otherwise Liverpool Central, however you might get bored very quickly as when you photo one 507/508 as that's the whole fleet.
As suggested by others, Sandhills station is on the surface & all Northern Line services pass through there. Kirkdale depot is by the next station along the Ormskirk/Kirkby lines which usually has a few 507/508's loitering with intent to work!
For the Wirral Line, there's no one spot above ground where you can see all of the services. Either any station between Birkenhead Central to Hooton on the Chester/Ellesmere Port route, or Birkenhead Park and or North on the West Kirkby/New Brighton routes.
Some units are stabled at West Kirkby, New Brighton, Birkenhead North depot and Rock Ferry at certain times of the day too.