They are really accessory movements. I don't consider either one to be a real developer of strength. I program them into a "work capacity" context on lower body days, often in circuit fashion with some other exercises. Step Ups are mainly a concentric-only exercise that can actually help dissipate soreness when programmed a few days after heavier squats / deads. Lunges actually create a lot of new soreness so I'd only recommend them if you are in pretty good shape and they aren't going to create severe DOMS. Either way, start with a lower volume and slowly build over time. I don't program either exercise heavy, especially the lunges.