With slides, the low-contrast lens will not reach the level of "blackness of the blacks" of the other lens. That will immediately appear as a "less lively" image to the eye when the two are compared. Colours will appear less saturated. In my experience perception of colour saturation is well dependent from overall contrast. The distance between black and white point determines how "lively" an image is.
As said, the soft-contrast lens (or at least the non-coated lens) will probably also lack some acutance, or "micro-contrast", which is perceived as "sharpness".
The simple, real-life test mentioned in this thread could be made, perhaps, by taking pictures with the same camera, same light conditions, same exposure, different lenses and slide film, and then measuring the difference between higher density and lower density points, which gives the "overall contrast".