I don't know a lot about human anatomy, but it seems to me that both my glutes and my quads get quite a workout with both exercises. In any event, I know that I can't seem to do leg presses followed by lunges. I'm back on the pyramid workouts again now, and PLB starts with step-ups and goes right into lunges. I can't do it that way. Instead, I tack the step-ups onto the day when I do the stability ball work. So I have one day when I do all the standing exercises except for step-ups, and another day when I start with step-ups and then do the floor work. I'm not sure if the workouts are as effective this way, but at least I get everything in.
As for you Alexis, although I think the exercises are quite similar, I do think you should practice your form on lunges until you get them right. I used to be off balance a lot, but haven't had that problem in a long time. Try to notice if you are spreading your legs too far. Try bringing them in a little closer to each other. Also, try using dumbbells instead of the barbell and see if they make you feel more in control.
As for your glasses, I'm at a loss. Mine don't slip at all when I'm doing lunges. Are you sweating from the cardio part of your workout?
-Nancy