I played both quite a lot but ı find myself being burned from mercenaries. With campaigns, ı dont feel this way cause it's much more fun and addicting to play through it while watching the cutscenes. I played the campaigns through different orders.
Right now ı'm playing through this order:
L1, C1, J1, A1, L2, C2, J2, A2, L3, C3, J3, A3, L4, C4, J4, A4, L5, C5, J5, A5.
I usually play 4 or 5 chapters one day. I also play ada's campaign again after finishing the order cause there's a carla mod ı found, ı like playing as ada while wearing her twin's outfit, particularly the scarf.
For 1st 3 campaigns, ı usually play them as helena, piers and sherry. Though during marathons, ı also play as leon, chris and jake.