You have a choice of north vs. south.

The service offering is the same. North is closer to (north) security, so easier to access; south is larger and can feel more private when it's relatively quiet (but both can be a bit of a zoo if rammed).

If the plane is going from a 'B' gate then there is also a business lounge on the 'B' concourse, which is small and quite pleasant and makes for an easier boarding experience. However LHR announces its gates very late so it can be hard to get the best out of it.