If it's BANANAS you're slicing, then either position is fine. If it's a marital aid you're looking for, I give this one a thumbs down. The ER cost us WAY more than the cost of the Hutzler 571. It's just not cost effective.
Works best if you stand the banana upright by balancing it on its end and then quickly apply the banana slicer with one hand and catch the slices with the other hand. Note that this applies only to the manual model banana slicer. The AC and 12 volt models work differently.
Unfortunately, in my experience, the bananas have always been lying down. I hope to experiment with erect bananas at some point, though until you mentioned it, I hadn't given it much thought. Perhaps I shall experiement--maybe if I play around with the banana, it will stand up. Although, if I try to hard--to say nothing of beating it--it might just end up nothing but mush anyway, or, if unpeeled, something might start oozing out of it. I don't honestly know. I will have to play around with it to find out. Get back to me in about three minutes, and perhaps I will have an answer for you.