The I-51 is a normal lens for 5x7. It is not a wide angle lens, won't cover 8x10, should be used on 5x7 and smaller formats. The I-37 is a normal lens for 8x10, also not a wide angle lens, should be used on 8x10 and smaller formats.
Get and use the one that's appropriate for the size of ambrotype you want to make.
Thank you Dan, if both of them are comparatively equal, I think I will get I-37, Im interested about 8x10 format.