Lunges are dynamic forward steps. The split squat does not include the forward step. Your legs start in a sagittal split position. A few minutes search on Youtube will make this difference obvious. As for weight to use. This is highly individual so I can’t tell you. I will tell you that lunges being dynamic place a high eccentric load on the glutes and hamstrings so start with body weight only for the first few workouts to get used to this movement. D0 not do the lunges too aggressively either or at best you will be very sore for a few days and risk hurting yourself.